MEMs display apparatus

ABSTRACT

This invention relates to display apparatuses having an array of light modulators and a plurality of spacers distributed within the interior of the array. The display apparatus may also include a reflective aperture layer disposed on a front facing surface of a substrate included in the display apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/656,307, entitled “Methods and Apparatus for ActuatingDisplays” and filed Jan. 19, 2007, which is a continuation-in-part ofU.S. patent application Ser. No. 11/251,035, entitled “Methods andApparatus for Actuating Displays” and filed Oct. 14, 2005, which is acontinuation-in-part of U.S. patent application Ser. No. 11/218,690,entitled “Methods and Apparatus for Spatial Light Modulation” and filedSep. 2, 2005, the latter two of which claim priority to and benefit of,U.S. Provisional Patent Application No. 60/676,053, entitled “MEMS BasedOptical Display” and filed on Apr. 29, 2005; and U.S. Provisional PatentApplication No. 60/655,827, entitled MEMS Based Optical Display Modules”and filed on Feb. 23, 2005. The entirety of each of these provisionaland non-provisional applications is incorporated by reference herein.

FIELD OF THE INVENTION

In general, the invention relates to the field of video displays, inparticular, the invention relates to mechanically actuated displayapparatus.

BACKGROUND OF THE INVENTION

Displays built from mechanical light modulators are an attractivealternative to displays based on liquid crystal technology. Mechanicallight modulators are fast enough to display video content with goodviewing angles and with a wide range of color and grey scale. Mechanicallight modulators have been successful in projection displayapplications. Backlit displays using mechanical light modulators havenot yet demonstrated sufficiently attractive combinations of brightnessand low power. There is a need in the art for fast, bright, low-poweredmechanically actuated displays. Specifically there is a need formechanically actuated displays that include bi-stable mechanisms andthat can be driven at low voltages for reduced power consumption.

SUMMARY OF THE INVENTION

According to one aspect of the invention, the display apparatus includesa first substrate, a second substrate, an array of MEMS lightmodulators, and a spacer. The array of MEMS light modulators are formedon one of the first and second substrates. The spacer is located withinthe interior of the array, integrally formed from or connected to thefirst substrate at a first end and connected to a second substrate at asecond end. The connection of the spacer to one of the first and secondsubstrates may include a connection to a stack of at least one thin filmdeposited on the substrate. The thin film may include one of areflective aperture layer, a light absorbing layer, and a color filter.

The spacer may be etched from the first substrate and/or from a filmdeposited on the first substrate. The spacer may form an electricalconnection between an electrical component on the first substrate to anelectrical component on the second substrate. The spacer may be formedfrom a polymer, a metal and/or an insulative material. The spacer mayinterfit with an aligning element formed on the second substrate. Thespacer may be between about 1 micron and about 10 microns tall.

The first substrate and the second substrate may both be substantiallyrigid. Alternatively, the first and second substrates may both besubstantially flexible. Or, one of the first substrate and the secondsubstrate is substantially rigid, and the other of the first substrateand the second substrate is substantially flexible. At least one of thefirst substrate and the second substrate is substantially transparent.

The first substrate or the second substrate may include a front surfaceof a display device. A light guide may be distinct from the first andsecond substrates. The spacer and a plurality of additional spacers maybe positioned within the array with a spacer density less than or equalto one spacer per four light modulators. The MEMS light modulators maycorrespond to respective display pixels, where each of the respectivedisplay pixels includes at least one spacer. A space between the firstand second substrates may form a gap, which may be filled with fluid,such as a lubricant.

The MEMS light modulators may be configured to selectively obstruct thepassage of light and may include shutter-based light modulators and/orelectrowetting-based light modulators. The MEMS light modulators mayselectively extract light from a guide. The spacer may limit a range ofmotion of a component in one of the MEMS light modulators.

According to another aspect of the invention, a display apparatusincludes a first substrate, a reflective aperture layer, and a pluralityof MEMS light modulators. The first substrate has a front-facing surfaceand a rear-facing surface. The reflective aperture layer includes aplurality of apertures disposed on the front-facing surface of the firstsubstrate. The plurality of MEMS light modulators modulate lightdirected towards the plurality of apertures to form an image.

The first substrate may include a light guide. Alternatively, a lightguide may be positioned behind the first substrate, where the reflectiveaperture layer may reflect light not passing through the plurality ofapertures back towards the light guide. In this case, the light guidemay be in intimate contact with the first substrate.

The plurality of MEMS light modulators may include shutter-based lightmodulators and/or electrowetting light modulators. An active matrixcontrol matrix may control the plurality of MEMS light modulators, wherethe active matrix may include at least one switch corresponding torespective MEMS light modulators. The first substrate may betransparent. The reflective aperture layer may be formed from one of amirror, a dielectric mirror, and a metallic film.

The MEMS light modulators may be disposed on the first substrate. Thefirst substrate may be positioned such that the reflective aperturelayer is proximate the plurality of light modulators. The plurality ofapertures may correspond to respective light modulators of the pluralityof light modulators.

In one embodiment, the first substrate is positioned with respect to alight guide so as to form a gap between the first substrate and thelight guide. A fluid, which may be air, may fill the gap. The fluid mayhave a first index of refraction and the light guide may have a secondindex of refraction, where the first index of refraction is less thanthe second index of refraction.

In another embodiment, a second substrate is positioned in front of thefront-facing surface of the first substrate. The plurality of MEMS lightmodulators may be formed on the second substrate or on a rear-facingsurface of the second substrate. A control matrix may be formed on thesecond substrate for controlling the plurality of MEMS light modulators.The second substrate may be transparent. Mechanically interlockingfeatures and/or an adhesive may maintain lateral alignment between thefirst and second substrates and may restrict relative lateral movementof the first and second substrates to less than 5 microns in anydimension. The first substrate may be positioned between the secondsubstrate and a light guide.

The first substrate may be positioned with respect to the secondsubstrate so as to form a gap between the first substrate and the secondsubstrate. The gap may be maintained by spacers and/or filled with aliquid or fluid, such as a lubricant. The fluid may have a first indexof refraction and the substrate may have a second index of refraction,where the first index of refraction is greater than or substantiallyequal to that of the second index of refraction.

BRIEF DESCRIPTION OF THE DRAWINGS

The system and methods may be better understood from the followingillustrative description with reference to the following drawings inwhich:

FIG. 1 is conceptual isometric view of a display apparatus, according toan illustrative embodiment of the invention;

FIGS. 2A-2B are top views of dual compliant beam electrodeactuator-based shutter assemblies for use in a display apparatus,according to an illustrative embodiment of the invention;

FIG. 3A is a diagram illustrating various compliant electrode shapessuitable for inclusion in dual compliant electrode actuator-basedshutter assemblies;

FIG. 3B is a diagram illustrating the incremental energy needed to movedual compliant electrode actuator-based shutter assemblies having theshapes illustrated in FIG. 3A;

FIGS. 3C-3F are top views of the compliant beam electrode actuator-basedshutter assembly of FIG. 2A in various stages of actuation;

FIGS. 4A and 4B are cross section views of a dual compliant electrodeactuator-based mirror-based light modulator in an active and an inactivestate, according to an illustrative embodiment of the invention;

FIG. 5 is a top view of a dual compliant beam electrode actuator-basedshutter assembly having a beam with thickness which varies along itslength, according to an illustrative embodiment of the invention;

FIG. 6 is an isometric view of a dual compliant beam electrodeactuator-based shutter assembly, according to an illustrative embodimentof the invention;

FIG. 7 is a top view of a dual compliant beam electrode actuator-basedshutter assembly including a return spring, according to an illustrativeembodiment of the invention;

FIG. 8 is a top view of a dual compliant beam electrode actuator-basedshutter assembly having separate open and close actuators, according toan illustrative embodiment of the invention;

FIG. 9 is a conceptual diagram of an active matrix array for controllingdual compliant electrode actuator based-light modulators, according toan illustrative embodiment of the invention;

FIG. 10 is a conceptual diagram of a second active matrix array forcontrolling dual compliant electrode actuator based-light modulators,according to an illustrative embodiment of the invention;

FIG. 11 is a cross sectional view of the dual compliant beam electrodeactuator-based shutter assembly of FIG. 8;

FIG. 12 is an energy diagram illustrating the energy characteristics ofvarious dual compliant electrode based shutter assemblies, according toan illustrative embodiment of the invention;

FIG. 13A is a top view of a bi-stable dual compliant beam electrodeactuator based-shutter assembly, according to an illustrative embodimentof the invention;

FIG. 13B shows the evolution of force versus displacement for abi-stable shutter assembly;

FIG. 14 is a top view of a second bi-stable dual compliant beamelectrode actuator based-shutter assembly, according to an illustrativeembodiment of the invention;

FIG. 15 is a top view of a tri-stable shutter assembly incorporatingdual compliant electrode actuators, according to an illustrativeembodiment of the invention;

FIGS. 16A-C are conceptual diagrams of another embodiment of a bi-stableshutter assembly, illustrating the state of the shutter assembly duringa change in shutter position, according to an illustrative embodiment ofthe invention;

FIG. 17A is a conceptual diagram of a bi-stable shutter assemblyincluding substantially rigid beams, according to an illustrativeembodiment of the invention;

FIG. 17B is a top view of a rotational bi-stable shutter assembly;

FIG. 18 is a conceptual diagram of a bi-stable shutter assemblyincorporating thermoelectric actuators, according to an illustrativeembodiment of the invention;

FIG. 19 is a conceptual diagram of a passive matrix array forcontrolling bi-stable shutter assemblies, according to an illustrativeembodiment of the invention;

FIGS. 20A and 20B are conceptual tiling diagrams for arranging shutterassemblies in a display apparatus;

FIG. 21 is cross-sectional view of a display apparatus, according to anillustrative embodiment of the invention;

FIGS. 22A and 22B are top views of the shutter assembly of FIG. 8 inopen and closed states, respectively, according to an illustrativeembodiment of the invention;

FIGS. 23A-23D are cross sectional views of shutter assemblies havingshutters, which, when in a closed position, overlap apertures formed inan adjacent reflective surface, according to an illustrative embodimentof the invention;

FIG. 24 is a cross sectional view of a first electrowetting-based lightmodulation array, according to an illustrative embodiment of theinvention;

FIG. 25 is a cross sectional view of a second electrowetting-based lightmodulation array, according to an illustrative embodiment of theinvention; and

FIG. 26 is a cross sectional view of a third electrowetting-based lightmodulation array, according to an illustrative embodiment of theinvention.

FIGS. 27A and 27B are cross sectional views of an aperture plate,according to an illustrative embodiment of the invention.

FIG. 28 is a cross sectional view of a display assembly, according to anillustrative embodiment of the invention.

FIG. 29A is a cross sectional view of a display assembly, according toan illustrative embodiment of the invention.

FIG. 29B is a cross sectional view of a display assembly, according toan illustrative embodiment of the invention.

FIGS. 30A and 30B are perspective views of a shutter assembly in openand closed states, respectively, according to an illustrative embodimentof the invention.

FIG. 31 is a cross sectional view of a display assembly, according to anillustrative embodiment of the invention.

FIG. 32 is a cross sectional view of a display assembly, according to anillustrative embodiment of the invention.

DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

FIG. 1 is an isometric view of a display apparatus 100, according to anillustrative embodiment of the invention. The display apparatus 100includes a plurality of light modulators, in particular, a plurality ofshutter assemblies 102 a-102 d (generally “shutter assemblies 102”)arranged in rows and columns. In general, a shutter assembly 102 has twostates, open and closed (although partial openings can be employed toimpart grey scale). Shutter assemblies 102 a and 102 d are in the openstate, allowing light to pass. Shutter assemblies 102 b and 102 c are inthe closed state, obstructing the passage of light. By selectivelysetting the states of the shutter assemblies 102 a-102 d, the displayapparatus 100 can be utilized to form an image 104 for a projection orbacklit display, if illuminated by lamp 105. In another implementationthe apparatus 100 may form an image by reflection of ambient lightoriginating from the front of the apparatus. In the display apparatus100, each shutter assembly 102 corresponds to a pixel 106 in the image104.

Each shutter assembly 102 includes a shutter 112 and an aperture 114. Toilluminate a pixel 106 in the image 104, the shutter 112 is positionedsuch that it allows light to pass, without any significant obstruction,through the aperture 114 towards a viewer. To keep a pixel 106 unlit,the shutter 112 is positioned such that it obstructs the passage oflight through the aperture 114. The aperture 114 is defined by anopening patterned through a reflective or light-absorbing material ineach shutter assembly 102.

In alternative implementations, a display apparatus 100 includesmultiple shutter assemblies 102 for each pixel 106. For example, thedisplay apparatus 100 may include three color-specific shutterassemblies 102. By selectively opening one or more of the color-specificshutter assemblies 102 corresponding to a particular pixel 106, thedisplay apparatus 100 can generate a color pixel 106 in the image 104.In another example, the display apparatus 100 includes two or moreshutter assemblies 102 per pixel 106 to provide grayscale in an image104. In still other implementations, the display apparatus 100 mayinclude other forms of light modulators, such as micromirrors, filters,polarizers, interferometric devices, and other suitable devices, insteadof shutter assemblies 102 to modulate light to form an image.

The shutter assemblies 102 of the display apparatus 100 are formed usingstandard micromachining techniques known in the art, includinglithography; etching techniques, such as wet chemical, dry, andphotoresist removal; thermal oxidation of silicon; electroplating andelectroless plating; diffusion processes, such as boron, phosphorus,arsenic, and antimony diffusion; ion implantation; film deposition, suchas evaporation (filament, electron beam, flash, and shadowing and stepcoverage), sputtering, chemical vapor deposition (CVD), plasma enhancedCVD, epitaxy (vapor phase, liquid phase, and molecular beam),electroplating, screen printing, and lamination. See generally Jaeger,Introduction to Microelectronic Fabrication (Addison-Wesley PublishingCo., Reading Mass. 1988); Runyan, et al., Semiconductor IntegratedCircuit Processing Technology (Addison-Wesley Publishing Co., ReadingMass. 1990); Proceedings of the IEEE Micro Electro Mechanical SystemsConference 1987-1998; Rai-Choudhury, ed., Handbook of Microlithography,Micromachining & Microfabrication (SPIE Optical Engineering Press,Bellingham, Wash. 1997).

More specifically, multiple layers of material (typically alternatingbetween metals and dielectrics) are deposited on top of a substrateforming a stack. After one or more layers of material are added to thestack, patterns are applied to a top most layer of the stack markingmaterial either to be removed from, or to remain on, the stack. Variousetching techniques, including wet or dry etches or reactive ion etching,are then applied to the patterned stack to remove unwanted material. Theetch process may remove material from one or more layers of the stackbased on the chemistry of the etch, the layers in the stack, and theamount of time the etch is applied. The manufacturing process mayinclude multiple iterations of layering, patterning, and etching.

In one implementation the shutter assemblies 102 are fabricated upon atransparent glass or plastic substrate. This substrate may be made anintegral part of a backlight which acts to evenly distribute theillumination from lamp 105 before the light exits through apertures 114.Alternatively, and optionally, the transparent substrate may be placedon top of a planar light guide, wherein the array of shutter assemblies102 act as light modulation elements in the formation of an image. Inone implementation the shutter assemblies 102 are fabricated inconjunction with or subsequent to the fabrication of a thin filmtransistor (TFT) array on the same glass or plastic substrate. The TFTarray provides a switching matrix for distribution of electrical signalsto the shutter assemblies.

The process also includes a release step. To provide freedom for partsto move in the resulting device, sacrificial material is interdisposedin the stack proximate to material that will form moving parts in thecompleted device. An etch removes much of the sacrificial material,thereby freeing the parts to move.

After release, one or more of the surfaces of the shutter assembly maybe insulated so that charge does not transfer between moving parts uponcontact. This can be accomplished by thermal oxidation and/or byconformal chemical vapor deposition of an insulator such as Al2O3,Cr2O3, TiO2, TiSiO4, HfO2, HfSiO4, V2O5, Nb2O5, Ta2O5, SiO2, or Si3N4 orby depositing similar materials using techniques such as atomic layerdeposition and others. The insulated surfaces are chemically passivatedto prevent problems such as stiction between surfaces in contact bychemical conversion processes such as fluoridation, silanization, orhydrogenation of the insulated surfaces.

Dual compliant electrode actuators make up one suitable class ofactuators for driving the shutters 112 in the shutter assemblies 102. Adual compliant beam electrode actuator, in general, is formed from twoor more at least partially compliant beams. At least two of the beamsserve as electrodes (also referred to herein as “beam electrodes”). Inresponse to applying a voltage across the beam electrodes, the beamelectrodes are attracted to one another from the resultant electrostaticforces. Both beams in a dual compliant beam electrode are, at least inpart, compliant. That is, at least some portion of each of the beams canflex and/or bend to aid in the beams being brought together. In someimplementations the compliance is achieved by the inclusion of flexuresor pin joints. Some portion of the beams may be substantially rigid orfixed in place. Preferably, at least the majority of the length of thebeams are compliant.

Dual compliant electrode actuators have advantages over other actuatorsknown in the art. Electrostatic comb drives are well suited foractuating over relatively long distances, but can generate onlyrelatively weak forces. Parallel plate or parallel beam actuators cangenerate relatively large forces but require small gaps between theparallel plates or beams and therefore only actuate over relativelysmall distances. R. Legtenberg et. al. (Journal ofMicroelectromechanical Systems v. 6, p. 257, 1997) demonstrated how theuse of curved electrode actuators can generate relatively large forcesand result in relatively large displacements. The voltages required toinitiate actuation in Legtenberg, however, are still substantial. Asshown herein such voltages can be reduced by allowing for the movementor flexure of both electrodes.

In a dual compliant beam electrode actuator-based shutter assembly, ashutter is coupled to at least one beam of a dual compliant beamelectrode actuator. As one of the beams in the actuator is pulledtowards the other, the pulled beam moves the shutter, too. In doing so,the shutter is moved from a first position to a second position. In oneof the positions, the shutter interacts with light in an optical pathby, for example, and without limitation, blocking, reflecting,absorbing, filtering, polarizing, diffracting, or otherwise altering aproperty or path of the light. The shutter may be coated with areflective or light absorbing film to improve its interferentialproperties. In the second position, the shutter allows the light to passby, relatively unobstructed.

FIGS. 2A and 2B are diagrams of two embodiments of cantilever dualcompliant beam electrode actuator based-shutter assemblies for use in adisplay apparatus, such as display apparatus 100. More particularly,FIG. 2A depicts a cantilever dual compliant beam electrodeactuator-based shutter assembly 200 a. The shutter assembly 200 amodulates light to form an image by controllably moving a shutter 202 ain and out of an optical path of light. In one embodiment, the opticalpath begins behind a surface 204 a, to which the shutter 202 a isattached. The surface 204 a is illustrated as a boundary line. However,the surface 204 a extends beyond the space delimited by the boundaryline. Similar boundary lines are used in other figures and may alsoindicate surfaces which extend beyond the space delimited by theboundary line. The light passes through an aperture 206 a in the surface204 a towards a viewer or towards a display screen. In anotherembodiment, the optical path begins in front of the surface 204 a and isreflected back to the viewer from the surface of the aperture 206 a.

The shutter 202 a of the shutter assembly 200 a is formed from a solid,substantially planar, body. The shutter 202 a can take virtually anyshape, either regular or irregular, such that in a closed position theshutter 202 a sufficiently obstructs the optical path through theaperture 206 a in the surface 204 a. In addition, the shutter 202 a musthave a width consistent with the width of the aperture, that, in theopen position (as depicted), sufficient light can pass through theaperture 206 a in the surface 204 a to illuminate a pixel, or contributeto the illumination of a pixel, in the display apparatus.

The shutter 202 a couples to one end of a load beam 208 a. A load anchor210 a, at the opposite end of the load beam 208 a physically connectsthe load beam 208 a to the surface 204 a and electrically connects theload beam 208 a to driver circuitry in the surface 204 a. Together, theload 208 a beam and load anchor 210 a serve as a mechanical support forsupporting the shutter 202 a over the surface 204 a.

The shutter assembly 200 a includes a pair of drive beams 212 a and 214a, one located along either side of the load beam 210 a. Together, thedrive beams 212 a and 214 a and the load beam 210 a form an actuator.One drive beam 212 a serves as a shutter open electrode and the otherdrive beam 214 a serves as a shutter close electrode. Drive anchors 216a and 218 a located at the ends of the drive beams 212 a and 214 aclosest to the shutter 202 a physically and electrically connects eachdrive beam 212 a and 214 a to the surface 204 a. In this embodiment, theother ends and most of the lengths of the drive beams 212 a and 214 aremain unanchored or free. The free ends of the drive beams 212 a and214 a are closer to the anchored end of the load beam 208 a than theanchored ends of the drive beams 212 a and 214 a are to the shutter endof the load beam 208 a.

The load beam 208 a and the drive beams 212 a and 214 a are compliant.That is, they have sufficient flexibility and resiliency that they canbe bent out of their unstressed (“rest”) position or shape to at leastsome useful degree, without fatigue or fracture. As the load beam 208 aand the drive beams 212 a and 214 a are anchored only at one end, themajority of the lengths of the beams 208 a, 212 a, and 214 a is free tomove, bend; flex, or deform in response to an applied force. Theoperation of the cantilever dual compliant beam electrode actuatorbased-shutter assembly 200 a is discussed further below in relation toFIG. 3.

FIG. 2B is a second illustrative embodiment of a cantilever dualcompliant beam electrode actuator-based shutter assembly 200 b. Like theshutter assembly 200 a, the shutter assembly 200 b includes a shutter202 b, coupled to a load beam 208 b, and two drive beams 212 b and 214b. The shutter 202 b is positioned in between its fully open positionand its fully closed position. The load beam 208 b and the drive beams212 b and 214 b, together, form an actuator. Drive anchors 210 b, 216 band 218 b, coupled to each end of the beams connect the beams to asurface 204 b. In contrast to the shutter assembly 200 a, the shutter ofshutter assembly 200 b includes several shutter apertures 220, in theform of slots. The surface 204 b, instead of only having one aperture,includes one surface aperture 206 b corresponding to each shutteraperture 220. In the open position, the shutter apertures 220substantially align with the apertures 206 b in the surface 204 b,allowing light to pass through the shutter 202 b. In the closedposition, the surface apertures 206 b are obstructed by the remainder ofthe shutter 202 b, thereby preventing the passage of light.

Changing the state of a shutter assembly that includes multiple shutterapertures with a corresponding number of surface apertures requires lessshutter movement than changing the state of a shutter assemblyincorporating a solid shutter and single surface aperture, while stillproviding for the same aperture area. Reduced required motioncorresponds to lower required actuation voltage. More particularly, adecrease in required motion by ⅓ reduces the necessary actuation voltageof the actuator by a factor of about ⅓. Reduced actuation voltagefurther corresponds to reduced power consumption. Since the totalaperture area for either shutter assembly is about the same, eachshutter assembly provides a substantially similar brightness.

In other implementations, the shutter apertures and correspondingsurface apertures have shapes other than slots. The apertures may becircular, polygonal or irregular. In alternative implementations, theshutter may include more shutter apertures than there are surfaceapertures in the shutter assembly. In such implementations, one or moreof the shutter apertures may serve as a filter, such as color filter.For example, the shutter assembly may have three shutter apertures forevery surface aperture, each shutter aperture including a red, blue, orgreen colored filter.

FIGS. 3A and 3B are diagrams illustrating the relationship between thedisplacement at the end of the load beam and the relative voltage neededto move the load beam closer to the drive beam. The displacement thatcan be achieved at any given voltage depends, at least in part, on thecurvature or shape of the drive beam, or more precisely, on how theseparation, d, and the bending stress along the drive beam and the loadbeam varies as a function of position x along the load beam A separationfunction d(x), shown in FIG. 3A can be generalized to the form ofd=ax^(n), where y is the distance between the beams. For example, ifn=1, the distance between drive electrode and load electrode increaselinearly along the length of the load electrode. If n=2, the distanceincreases parabolically. In general, assuming a constant voltage, as thedistance between the compliant electrodes decreases, the electrostaticforce at any point on the beams increases proportional to 1/d. At thesame time, however, any deformation of the load beam which mightdecrease the separation distance may also result in a higher stressstate in the beam. Below a minimum threshold voltage a limit ofdeformation will be reached at which any electrical energy released by acloser approach of the electrodes is exactly balanced by the energywhich becomes stored in the deformation energy of the beams.

As indicated in the diagram 3B, for actuators having separationfunctions in which n is less than or equal to 2, the application of aminimum actuation voltage (V₂) results in a cascading attraction of theload beam to the drive beam without requiring the application of ahigher voltage. For such actuators, the incremental increase inelectrostatic force on the beams resulting from the load beam gettingcloser to the drive beam is greater than the incremental increase instress on the beams needed for further displacement of the beams.

For actuators having separation functions in which x is greater than 2,the application of a particular voltage results in a distinct partialdisplacement of the load electrode. That is, the incremental increase inelectrostatic force on the beams resulting from a particular decrease inseparation between the beams, at some point, fails to exceed theincremental deformation force needed to be imparted on the load beam tocontinue reducing the separation. Thus, for actuators having separationfunctions having n greater than 2, the application of a first voltagelevel results in a first corresponding displacement of the loadelectrode. A higher voltage results in a greater correspondingdisplacement of the load electrode. How the shapes and relativecompliance of thin beam electrodes effects actuation voltage isdiscussed in more detail in the following references: (R. Legtenberg et.al., Journal of Microelectromechanical Systems v. 6, p. 257 (1997) andJ. Li et. al. Transducers '03, The 12^(th) International Conference onSolid State Sensors, Actuators, and Microsystems, p. 480 (2003), each ofwhich is incorporated herein by reference.

Referring back to FIGS. 2A and 2B, a display apparatus incorporating theshutter assemblies 202 a and 202 b actuates, i.e., changes the positionof the shutter assemblies 202 a and 202 b, by applying an electricpotential, from a controllable voltage source, to one of the drive beams212 a, 212 b, 214 a, or 214 b via its corresponding drive anchor 216 a,216 b, 218 a, or 218 b, with the load beam 208 a or 208 b beingelectrically coupled to ground, resulting in a voltage across the beams208 a, 208 b, 212 a, 212 b, 214 a, 214 b. The controllable voltagesource, such as an active matrix array driver, is electrically coupledto load beam 208 a or 208 b via an active matrix array (see FIGS. 9 and10 below). The display apparatus may instead apply an electric potentialto the load beam 208 a or 208 b via the load anchor 210 a or 210 b ofthe shutter assembly 202 a or 202 b to increase the voltage. Anelectrical potential difference between the drive beams and the loadbeams, regardless of sign or ground potential, will generate anelectrostatic force between the beams.

With reference back to FIG. 3, the shutter assembly 200 a of FIG. 2A hasa second order separation function (i.e., n=2). Thus, if the voltage orpotential difference between the beams 208 a and 212 a or 214 a of theshutter assembly 202 a at their point of least separation exceeds theminimum actuation voltage (V₂) the deformation of the beams 208 a and212 a or 214 a cascades down the entire lengths of the beams 208 a and212 a or 214 a, pulling the shutter end of the load beam 208 a towardsthe anchored end of the drive beam 212 a or 214 a. The motion of theload beam 208 a displaces the shutter 202 a such that it changes itsposition from either open to closed, or visa versa, depending on towhich drive beam 212 a or 214 a the display apparatus applied thepotential. To reverse the position change, the display apparatus ceasesapplication of the potential to the energized drive beam 212 a or 214 a.Upon the display apparatus ceasing to apply the potential, energy storedin the form of stress on the deformed load beam 208 a restores the loadbeam 208 a to its original or rest position. To increase the speed ofthe restoration and to reduce any oscillation about the rest position ofthe load beam 208 a, the display apparatus may return the shutter 202 ato its prior position by applying an electric potential to the opposingdrive beam 212 a or 214 a.

The shutter assemblies 200 a and 200 b, as well as shutter assemblies500 (see FIG. 5 below), 600 (see FIG. 6 below), 700 (see FIG. 7 below)and 800 (see FIG. 8 below) have the property of being electricallybi-stable. Generally, this is understood to encompass, although not belimited to, devices wherein the electrical potential V₂ that initiatesmovement between open and closed states is generally greater than theelectrical potential (V₁) required to keep the shutter assembly in astable state. Once the load beam 208 a and one of the drive beams are incontact, a substantially greater electrical force is to be applied fromthe opposing drive beam to move or separate the load beam, suchelectrical force being greater than would be necessary if the load beam208 a were to begin in a neutral or non-contact position. The bistabledevices described herein may employ a passive matrix driving scheme forthe operation of an array of shutter assemblies such as 200 a. In apassive matrix driving sequence it is possible to preserve an image bymaintaining a stabilization voltage V₁ across all shutter assemblies(except those that are being actively driven to a state change). With noor substantially no electrical power required, maintenance of apotential V₁ between the load beam 208 a and drive beam 212 a or 214 ais sufficient to maintain the shutter assembly in either its open orclosed states. In order to effect a switching event the voltage betweenload beam 208 a and the previously affected drive beam (for instance 212a) is allowed to return from V₁ to zero while the voltage between theload beam 208 a and the opposing beam (for instance 212 b) is brought upto the switching voltage V₂.

In FIG. 2B, the actuator has a third order separation function (i.e.,n=3). Thus applying a particular potential to one of the drive beams 212b or 214 b results in an incremental displacement of the shutter 202 b.The display apparatus takes advantage of the ability to incrementallydisplace the shutter 202 b to generate a grayscale image. For example,the application of a first potential to a drive beam 212 or 214 bdisplaces the shutter 202 b to its illustrated position, partiallyobstructing light passing through the surface apertures 206 b, but stillallowing some light to pass through the shutter 202 b. The applicationof other potentials results in other shutter 202 b positions, includingfully open, fully closed, and other intermediate positions between fullyopen and fully closed. In such fashion electrically analog drivecircuitry may be employed in order to achieve an analog grayscale image.

FIGS. 3C through 3F demonstrate the stages of motion of the load beam208 a, the shutter close electrode 214 a, and the shutter 202 a of theshutter assembly 200 a of FIG. 2A. The initial separation between thecompliant beams 208 a and 214 a fits a second order separation function.FIG. 3C shows the load beam 208 a in a neutral position with no voltageapplied. The aperture 206 a is half-covered by the shutter 202 a.

FIG. 3D demonstrates the initial steps of actuation. A small voltage isapplied between the load beam 208 a and the shutter close electrode 214a. The free end of the shutter close electrode 214 a has moved to makecontact with the load beam 208 a.

FIG. 3E shows the shutter assembly 200 a at a point of actuation afterthe shutter 202 a begins to move towards the shutter close electrode 214a.

FIG. 3F shows the end state of actuation of the shutter assembly 200 a.The voltage has exceeded the threshold for actuation. The shutterassembly 200 a is in the closed position. Contact is made between theload beam 208 a and the shutter close electrode 214 a all along itslength.

FIG. 4A is a first cross sectional diagram of dual compliant electrodemirror-based light modulator 400 for inclusion in a display apparatus,such as display apparatus 100, instead of, or in addition to, theshutter assemblies 102. The mirror-based-based light modulator 400includes a mechanically compliant reflection platform 402. At least aportion of the reflection platform 402 is itself reflective or is coatedwith or is connected to a reflective material.

The reflection platform 402 may or may not be conductive. Inimplementations in which the reflection platform 402 is conductive, thereflection platform serves as a load electrode for the mirror-basedlight modulator 400. The reflection platform 402 is physically supportedover, and is electrically coupled to, a substrate 404 via a compliantsupport member 406. If the reflection platform 402 is formed from anon-conductive material, the reflection platform 402 is coupled to acompliant conductive load beam or other form of compliant loadelectrode. A compliant support member 406 physically supports thecombined reflection platform 402 and electrode over the substrate 404.The support member 406 also provides an electrical connection from theelectrode to the substrate 404.

The mirror-based light modulator 400 includes a second compliantelectrode 408, which serves a drive electrode 408. The drive electrode408 is supported between the substrate 404 and the reflection platform402 by a substantially rigid second support member 410. The secondsupport member 410 also electrically connects the second compliantelectrode 408 to a voltage source for driving the mirror-based lightmodulator 400.

The mirror-based light modulator 400 depicted in FIG. 4A is in restposition in which neither of the electrodes 402 or 408 carry apotential. FIG. 4B depicts the mirror-based light modulator 400 in anactivated state. When a potential difference is generated between thedrive electrode 408 and the load electrode 402 (be it the reflectiveplatform 402 or an attached load beam), the load electrode 402 is drawntowards the drive electrode 408, thereby bending the compliant supportbeam 406 and angling the reflective portion of the reflection platform402 to be at least partially transverse to the substrate 404.

To form an image, light 412 is directed at an array of mirror-basedlight modulators 400 at a particular angle. Mirror-based lightmodulators 400 in their rest states reflect the light 412 away from theviewer or the display screen, and mirror-based light modulators in theactive state reflect the light 412 towards a viewer or a display screen,or visa versa.

FIG. 5 is a diagram of another cantilever dual compliant beam electrodeactuator-based shutter assembly 500. As with the shutter assemblies 200a and 200 b, the shutter assembly 500 includes a shutter 502 coupled toa compliant load beam 504. The compliant load beam 504 is thenphysically anchored to a surface 506, and electrically coupled toground, at its opposite end via a load anchor 508. The shutter assembly500 includes only one compliant drive beam 510, located substantiallyalongside the load beam 504. The drive beam 510, in response to beingenergized with an electric potential from a controllable voltage sourcedraws the shutter 502 from a first position (in which the load beam 504is substantially unstressed) in a plane substantially parallel to thesurface, to a second position in which the load beam 504 is stressed.When the potential is removed, the stored stress in the load beam 504restores the load beam 504 to its original position.

In addition, in comparison to the shutter assemblies 202 a and 202 b,the load beam 504 has a width which varies along its length. The loadbeam 504 is wider near its anchor 508 than it is nearer to the shutter502. In comparison to the shutter assemblies 202 a and 202 b and becauseof its tailored width, the load beam 504 typically has an overallgreater stiffness. Shutter assemblies incorporating stiffer beamstypically require higher voltages for actuation, but in return, allowfor higher switching rates. For example, the shutter assemblies 202 aand 202 b may be switched up to about 10 kHz, while the stiffer shutterassembly 500 may be switched up to about 100 kHz.

FIG. 6 is a diagram of a shutter assembly 600 incorporating two dualcompliant electrode beam actuators 602 (“actuators 602”), according toan illustrative embodiment of the invention. The shutter assembly 600includes a shutter 604. The shutter 604 may be solid, or it may includeone or more shutter apertures as described in relation to FIG. 2B. Theshutter 604 couples on one side to the beam actuators 602. Together, theactuators 602 move the shutter transversely over a surface in plane ofmotion which is substantially parallel to the surface.

Each actuator 602 includes a compliant load member 606 connecting theshutter 604 to a load anchor 608. The compliant load members 606 eachinclude a load beam 610 and an L bracket 612. The load anchors 608 alongwith the compliant load members 606 serve as mechanical supports,keeping the shutter 604 suspended proximate to the surface. The loadanchors 608 physically connect the compliant load members 606 and theshutter 604 to the surface and electrically connect the load beams 610of the load members 606 to ground. The coupling of the shutter 604 fromtwo positions on one side of the shutter 604 to load anchors 608 inpositions on either side of the shutter assembly 600 help reducetwisting motion of the shutter 604 about its central axis 614 duringmotion.

The L brackets 612 reduce the in-plane stiffness of the load beam. 610.That is, the L brackets 612 reduce the resistance of actuators 602 tomovement in a plane parallel to the surface (referred to as “in-planemovement” 615), by relieving axial stresses in the load beam.

Each actuator 602 also includes a compliant drive beam 616 positionedadjacent to each load beam 610. The drive beams 616 couple at one end toa drive beam anchor 618 shared between the drive beams 616. The otherend of each drive beam 616 is free to move. Each drive beam 616 iscurved such that it is closest to the load beam 610 near the free end ofthe drive beam 616 and the anchored end of the load beam 610.

In operation, a display apparatus incorporating the shutter assembly 600applies an electric potential to the drive beams 616 via the drive beamanchor 618. As a result of a potential difference between the drivebeams 616 and the load beam 610, the free ends of the drive beams 616are pulled towards the anchored ends of the load beams 610 and theshutter ends of the load beams 610 are pulled toward the anchored endsof the drive beams 616. The electrostatic force draws the shutter 604towards the drive anchor 618. The compliant members 606 act as springs,such that when the electrical potentials are removed from the drivebeams 616, the load beams compliant members 606 push the shutter 604back into its initial position, releasing the stress stored in the loadbeams 610. The L brackets 612 also serve as springs, applying furtherrestoration force to the shutter 604.

In fabrication of shutter assemblies 200 through 800, as well as forshutter assemblies 1300 through 1800, it is preferable to provide arectangular shape for the cross section of the load beams (such as loadbeams 610) and the drive beams (such as drive beams 616). By providing abeam thickness (in the direction perpendicular to surface) which is 1.4times or more larger in dimension than the beam width (in a directionparallel to the surface) the stiffness of the load beam 610 will beincreased for out-of-plane motion 617 versus in-plane motion 615. Such adimensional and, by consequence, stiffness differential helps to ensurethat the motion of the shutter 604, initiated by the actuators 602, isrestricted to motion along the surface and across the surface aperturesas opposed to out-of-plane motion 617 which would a wasteful applicationof energy. It is preferable for certain applications that the crosssection of the load beams (such as 610) be rectangular as opposed tocurved or elliptical in shape. The strongest actuation force is achievedif the opposing beam electrodes have flat faces so that upon actuationthey can approach and touch each other with the smallest possibleseparation distance.

FIG. 7 is a diagram of a second shutter assembly 700 incorporating twodual compliant electrode beam actuators 702, according to anillustrative embodiment of the invention. The shutter assembly 700 takesthe same general form of the shutter assembly 600, other than itincludes a return spring 704. As with the shutter assembly 600, in theshutter assembly 700, two actuators 702 couple to a first side of ashutter 706 to translate the shutter 706 in a plane parallel to asurface over which the shutter is physically supported. The returnspring 704 couples to the opposite side of the shutter 706. The returnspring 704 also couples to the surface at a spring anchor 708, acting asan additional mechanical support. By physically supporting the shutter706 over the surface at opposite sides of the shutter 706, the actuators702 and the return spring 704 reduce motion of the shutter 706 out ofthe plane of intended motion during operation. In addition, the returnspring 704 incorporates several bends which reduce the in-planestiffness of the return spring 704, thereby further promoting in-planemotion over out-of-plane motion. The return spring 704 provides anadditional restoration force to the shutter 706, such that once anactuation potential is removed, the shutter 706 returns to its initialposition quicker. The addition of the return spring 704 increases onlyslightly the potential needed to initiate actuation of the actuators702.

FIG. 8 is a diagram of a shutter assembly 800 including a pair ofshutter open actuators 802 and 804 and a pair of shutter close actuators806 and 808, according to an illustrative embodiment of the invention.Each of the four actuators 802, 804, 806, and 808 take the form of adual compliant beam electrode actuator. Each actuator 802, 804, 806, and808 includes a compliant load member 810 coupling a shutter 812, at oneend, to a load anchor 814, at the other end. Each compliant load member810 includes a load beam 816 and an L bracket 818. Each actuator 802,804, 806, and 808 also includes a drive beam 820 with one end coupled toa drive anchor 822. Each pair of actuators 802/804 and 806/808 share acommon drive anchor 822. The unanchored end of each drive beam 820 ispositioned proximate to the anchored end of a corresponding compliantload member 810. The anchored end of each drive beam 820 is locatedproximate to the L bracket end of a corresponding load beam 816. In adeactivated state, the distance between a load beam 816 and itscorresponding drive beam 820 increases progressively from the anchoredend of the load beam 816 to the L bracket 818.

In operation, to open the shutter 812, a display apparatus incorporatingthe shutter assembly 800 applies an electric potential to the driveanchor 822 of the shutter open actuators 802 and 804, drawing theshutter 812 towards the open position. To close the shutter 812, thedisplay apparatus applies an electric potential to the drive anchor 822of the shutter close actuators 806 and 808 drawing the shutter 812towards the closed position. If neither pair of actuators 802/804 or806/808 are activated, the shutter 812 remains in an intermediateposition, somewhere between fully open and fully closed.

The shutter open actuators 802/804 and shutter closed actuators 806/808couple to the shutter 812 at opposite ends of the shutter. The shutteropen and closed actuators have their own load members 810, thus reducingthe actuation voltage of each actuator 802, 804, 806 and 808. Because ofthe electrical bi-stability described in reference to FIG. 3, it isadvantageous to find an actuation method or structure with more leveragefor separating the compliant load member 810 from a drive beam 820 withwhich it might be in contact. By positioning the open and closedactuators 802/804 and 806/808 on opposite sides of the shutter 812, theactuation force of the actuator-to-be-actuated is transferred to theactuator-to-be-separated through the shutter. The actuation force istherefore applied to the task of separation at a point close to theshutter (for instance near the L-bracket end of the load beam 816) whereits leverage will be higher.

For shutter assemblies such as in FIG. 8 typical shutter widths (alongthe direction of the slots) will be in the range of 20 to 800 microns.The “throw distance” or distance over which the shutter will movebetween open and closed positions will be in the range of 4 to 100microns. The width of the drive beams and load beams will be in therange of 0.2 to 40 microns. The length of the drive beams and load beamswill be in the range of 10 to 600 microns. Such shutter assemblies maybe employed for displays with resolutions in the range of 30 to 1000dots per inch.

Each of the shutter assemblies 200 a, 200 b, 500, 600, 700 and 800, andthe mirror-based light modulator 400, described above fall into a classof light modulators referred to herein as “elastic light modulators.”Elastic light modulators have one mechanically stable rest state. In therest state, the light modulator may be on (open or reflecting), off(closed or not reflecting), or somewhere in between (partially open orpartially reflecting). If the generation of a voltage across beams in anactuator forces the light modulator out of its rest state into amechanically unstable state, some level of voltage across the beams mustbe maintained for the light modulator to remain in that unstable state.

FIG. 9 is a diagram of an active matrix array 900 for controllingelastic light modulators 902 in a display apparatus. In particular, theactive matrix array 900 is suitable for controlling elastic lightmodulators 902, such as the mirror-based light modulator 400 orshutter-based light modulators 500, 600, and 700, that include only apassive restoration force. That is, these light modulators 902 requireelectrical activation of actuators to enter a mechanically unstablestate, but then utilize mechanical mechanisms, such as springs, toreturn to the rest state.

The active matrix array is fabricated as a diffused orthin-film-deposited electrical circuit on the surface of a substrate onwhich the elastic light modulators 902 are formed. The active matrixarray 900 includes a series of row electrodes 904 and column electrodes906 forming a grid like pattern on the substrate, dividing the substrateinto a plurality of grid segments 908. The active matrix array 900includes a set of drivers 910 and an array of non-linear electricalcomponents, comprised of either diodes or transistors that selectivelyapply potentials to grid segments 908 to control one or more elasticlight modulators 902 contained within the grid segments 908. The art ofthin film transistor arrays is described in Active Matrix Liquid CrystalDisplays: Fundamentals and Applications by Willem den Boer (Elsevier,Amsterdam, 2005).

Each grid segment 908 contributes to the illumination of a pixel, andincludes one or more elastic light modulators 902. In grid segments 908,including only a single elastic light modulator 902, the grid segment908 includes, in addition to the elastic light modulator 902, at leastone diode or transistor 912 and optionally a capacitor 914. Thecapacitor 914 shown in FIG. 9 can be explicitly added as a designelement of the circuit, or it can be understood that the capacitor 914represents the equivalent parallel or parasitic capacitance of theelastic light modulator. The emitter 916 of the transistor 912 iselectrically coupled, to either the drive electrode or the loadelectrode of the elastic light modulator 902. The other electrode of theactuator is coupled to a ground or common potential. The base 918 of thetransistor 912 electrically couples to a row electrode 904 controlling arow of grid segments. When the base 918 of the transistor receives apotential via the row electrode 904, current can run through thetransistor 912 from a corresponding column electrode 906 to generate apotential in the capacitor 914 and to apply a potential to the driveelectrode of the elastic light modulator 902 activating the actuator.

The active matrix array 900 generates an image, in one implementationby, one at a time, applying a potential from one of the drivers 910 to aselected row electrode 904, activating a corresponding row of gridsegments 908. While a particular row is activated, the display apparatusapplies a potential to the column electrodes corresponding to gridsegments in the active row containing light modulators which need to beswitched out of a rest state.

When a row is subsequently deactivated, a stored charge will remain onthe electrodes of the actuator 902 (as determined by the equivalentcapacitance of the actuator) as well as, optionally, on the parallelcapacitor 914 that can be designed into the circuit, keeping the elasticshutter mechanisms 902 in their mechanically unstable states. Theelastic shutter mechanism 902 remains in the mechanically unstable stateuntil the voltage stored in the capacitor 914 dissipates or until thevoltage is intentionally reset to ground potential during a subsequentrow selection or activation step.

FIG. 10 is diagram of another implementation of an active matrix array1000 for controlling elastic light modulators 1002 in a displayapparatus. In particular, the active matrix array 1000 is suitable forcontrolling elastic light modulators, such as shutter-based lightmodulators 200 a, 200 b, and 800, which include one set of actuators forforcing the light modulators from a rest state to a mechanicallyunstable state and a second set of actuators for driving the lightmodulators back to the rest state and possibly to a second mechanicallyunstable state. Active matrix array 1000 can also be used for drivingnon-elastic light modulators described further in relation to FIGS.12-20.

The active matrix array 1000 includes one row electrode 1004 for eachrow in the active matrix array 1000 and two column electrodes 1006 a and1006 b for each column in the active matrix array 1000. For example, fordisplay apparatus including shutter-based light modulators, one columnelectrode 1006 a for each column corresponds to the shutter openactuators of light modulators 1002 in the column. The other columnelectrode 1006 b corresponds to the shutter close actuators of the lightmodulators 1002 in the column. The active matrix array 1000 divides thesubstrate upon which it is deposited into grid sections 1008. Each gridsection 1008 includes one or more light modulators 1002 and at least twodiodes or transistors 1010 a and 1010 b and optionally two capacitors1012 a and 1012 b. The bases 1014 a and 1014 b of each transistor 1010 aand 1010 b are electrically coupled to a column electrode 1006 a or 1006b. The emitters 1016 a and 1016 b of the transistors 1010 a and 1010 bare coupled to a corresponding capacitor 1012 a or 1012 b and a driveelectrode of the light modulator(s) 1002 in the grid section 1008.

In operation, a driver applies a potential to a selected row electrode1004, activating the row. The active matrix array 1000 selectivelyapplies potentials to one of the two column electrodes 1006 a or 1006 bof each column in which the state of the light modulator(s) 1002 in thegrid section 1008 needs to be changed. Alternatively, the active matrixarray 1000 may also apply a potential to column electrodes 1006 a or1006 b for grid sections 1008 previously in an active state which are toremain in an active state.

For both active matrix arrays 900 and 1000, the drivers powering thecolumn electrodes, in some implementations, select from multiplepossible potentials to apply to individual column electrodes 1006 a and1006 b. The light modulator(s) 1002 in those columns can then be openedor closed different amounts to create grayscale images.

FIG. 11 is a cross sectional view of the shutter-assembly 800 of FIG. 8along the line labeled A-A′. Referring to FIGS. 8, 10, and 11, theshutter assembly 800 is built on substrate 1102 which is shared withother shutter assemblies of a display apparatus, such as displayapparatus 100, incorporating the shutter assembly 800. The voltagesignals to actuate the shutter assembly, are transmitted alongconductors in underlying layers of the shutter assembly. is The voltagesignals are controlled by an active matrix array, such as active matrixarray 1000. The substrate 1102 may support as many as 4,000,000 shutterassemblies, arranged in up to about 2000 rows and up to about 2000columns.

In addition to the shutter 812, the shutter open actuators 802 and 804,the shutter close actuators 806 and 808, the load anchors 814 and thedrive anchors 822, the shutter assembly 800 includes a row electrode1104, a shutter open electrode 1106, a shutter close electrode 1108, andthree surface apertures 1110. The depicted shutter assembly has at leastthree functional layers, which may be referred to as the row conductorlayer, the column conductor layer, and the shutter layer. The shutterassembly is preferably made on a transparent substrate such as glass orplastic. Alternatively the substrate can be made from an opaque materialsuch as silicon, as long as through holes are provided at the positionsof each of the surface apertures 1110 for the transmission of light. Thefirst metal layer on top of the substrate is the row conductor layerwhich is patterned into row conductor electrodes 1104 as well asreflective surface sections 1105. The reflective surface sections 1105reflect light passing through the substrate 1102 back through thesubstrate 1102 except at the surface apertures 1110. In someimplementations the surface apertures may include or be covered by red,green, or blue color filtering materials.

The shutter open electrode 1106 and the shutter close electrode 1108 areformed in a column conductor layer 1112 deposited on the substrate 1102,on top of the row conductor layer 1104. The column conductor layer 1112is separated from the row conductor layer 1104 by one or moreintervening layers of dielectric material or metal. The shutter openelectrode 1104 and the shutter close electrode 1106 of the shutterassembly 800 are shared with other shutter assemblies in the same columnof the display apparatus. The column conductor layer 1112 also serves toreflect light passing through gaps in the ground electrode 1104 otherthan through the surface apertures 1110. The row conductor layer 1104and the column conductor layer 1112 are between about 0.1 and about 2microns thick. In alternative implementations, the column conductor 1112layer can be located below the row conductor layer 1104. In anotheralternative implementation both the column conductor layer and the rowconductor layer may be located above the shutter layer.

The shutter 812, the shutter open actuators 802 and 804, the shutterclose actuators 806 and 808, the load anchors 814 and the drive anchors822 are formed from a third functional layer of the shutter assembly800, referred to as the shutter layer 1114. The actuators 802, 804, 806,and 808 are formed from a deposited metal, such as, without limitation,Au, Cr or Ni, or a deposited semiconductor, such as, without limitation,polycrystalline silicon, or amorphous silicon, or from single crystalsilicon if formed on top of a buried oxide (also known as silicon oninsulator). The beams of the actuators 802, 804, 806, and 808 arepatterned to dimensions of about 0.2 to about 20 microns in width. Theshutter thickness is typically in the range of 0.5 microns to 10microns. To promote the in-plane movement of the shutters (i.e. reducethe transverse beam stiffness as opposed to the out-of-plane stiffness),it is preferable to maintain a beam dimensional ratio of about at least1.4:1, with the beams being thicker than they are wide.

Metal or semiconductor vias electrically connect the row electrode 1104and the shutter open electrode 1106 and the shutter close electrode 1108of the column conductor layer 1112 to features on the shutter layer1114. Specifically, vias 1116 electrically couple the row electrode 1104to the load anchors 814 of the shutter assembly 800, keeping thecompliant load member 810 of the shutter open actuators 802 and 804 andthe shutter close actuators 806 and 808, as well as the shutter 812, atthe row conductor potential. Additional vias electrically couple theshutter open electrode 1106 to the drive beams 820 of the shutter openactuators 802 and 804 via the drive anchor 822 shared by the shutteropen actuators 802 and 804. Still other vias electrically couple theshutter close electrode 1108 to the drive beams 820 of the of theshutter close actuators 806 and 808 via the drive anchor 822 shared bythe shutter close actuators 806 and 808.

The shutter layer 1114 is separated from the column conductor layer 1112by a lubricant, vacuum or air, providing the shutter 812 freedom ofmovement. The moving pieces in the shutter layer 1114 are mechanicallyseparated from neighboring components (except their anchor points 814)in a release step, which can be a chemical etch or ashing process, whichremoves a sacrificial material from between all moving parts.

The diodes, transistors, and/or capacitors (not shown for purpose ofclarity) employed in the active matrix array may be patterned into theexisting structure of the three functional layers, or they can be builtinto separate layers that are disposed either between the shutterassembly and the substrate or on top of the shutter layer. Thereflective surface sections 1105 may be patterned as extensions of therow and column conductor electrodes or they can be patterned asfree-standing or electrically floating sections of reflective material.Alternatively the reflective surface sections 1105 along with theirassociated surface apertures 1110 can be patterned into a fourthfunctional layer, disposed between the shutter assembly and thesubstrate, and formed from either a deposited metal layer or adielectric mirror. Grounding conductors may be added separately from therow conductor electrodes in layer 1104. These separate groundingconductors may be required when the rows are activated throughtransistors, such as is the case with an active matrix array. Thegrounding conductors can be either laid out in parallel with the rowelectrodes (and bussed together in the drive circuits), or the groundingelectrodes can be placed into separate layers between the shutterassembly and the substrate.

In addition to elastic light modulators, display apparatus can includebi-stable light modulators, for example bi-stable shutter assemblies. Asdescribed above, a shutter in an elastic shutter assembly has onemechanically stable position (the “rest position”), with all othershutter positions being mechanically unstable. The shutter of abi-stable shutter assembly, on the other hand, has two mechanicallystable positions, for example, open and closed. Mechanically bi-stableshutter assemblies have the advantage that no voltage is required tomaintain the shutters in either the open or the closed positions.Bi-stable shutter assemblies can be further subdivided into two classes:shutter assemblies in which each stable position is substantiallyenergetically equal, and shutter assemblies in which one stable positionis energetically preferential to the other mechanically stable position.

FIG. 12 is a diagram 1200 of potential energy stored in three types ofshutter assemblies in relation to shutter position. The solid line 1202corresponds to an elastic shutter assembly. The first dashed line 1204corresponds to a bi-stable shutter assembly with equal energy stablestates. The second dashed line 1206 corresponds to a bi-stable shutterassembly with non-equal energy stable states. As indicated in the energydiagram 1200, the energy curves 1204 and 1206 for the two types ofbi-stable shutter assemblies each include two local minima 1208,corresponding to stable shutter positions, such as fully open 1210 andfully closed 1211. As illustrated, energy must be added to the assemblyin order to move its shutters out of the positions corresponding to oneof the local minima. For the bi-stable shutter assemblies withnon-equal-energy mechanically stable shutter positions, however, thework needed to open a shutter 1212 is greater than the work required toclose the shutter 1214. For the elastic shutter assembly, on the otherhand, opening the shutter requires work 1218, but the shutter closesspontaneously after removal of the control voltage.

FIG. 13A is a top view of a shutter layer 1300 of a bi-stable shutterassembly. The shutter layer 1300 includes a shutter 1302 driven by twodual compliant electrode actuators 1304 and 1306. The shutter 1302includes three slotted shutter apertures 1308. One dual compliantelectrode actuator 1304 serves as a shutter open actuator. The otherdual compliant electrode actuator 1306 serves as a shutter closeactuator.

Each dual compliant electrode actuator 1304 and 1306 includes acompliant member 1310 connecting the shutter 1302, at about its linearaxis 1312, to two load anchors 1314, located in the corners of theshutter layer 1300. The compliant members 1310 each include a conductiveload beam 1316, which may have an insulator disposed on part of, or theentirety of its surface. The load beams 1316 serve as mechanicalsupports, physically supporting the shutter 1302 over a substrate onwhich the shutter assembly is built. The actuators 1304 and 1306 alsoeach include two compliant drive beams 1318 extending from a shareddrive anchor 1320. Each drive anchor 1320 physically and electricallyconnects the drive beams 1318 to the substrate. The drive beams 1318 ofthe actuators 1304 and 1306 curve away from their corresponding driveanchors 1320 towards the points on the load anchors 1314 at which loadbeams 1316 couple to the load anchors 1314. These curves in the drivebeams 1318 act to reduce the stiffness of the drive beams, therebyhelping to decrease the actuation voltage.

Each load beam 1316 is generally curved, for example in a bowed (orsinusoidal) shape. The extent of the bow is determined by the relativedistance between the load anchors 1314 and the length of the load beam1316. The curvatures of the load beams 1316 provide the bi-stability forthe shutter assembly 1300. As the load beam 1316 is compliant, the loadbeam 1316 can either bow towards or away from the drive anchor 1320. Thedirection of the bow changes depending on what position the shutter 1302is in. As depicted, the shutter 1302 is in the closed position. The loadbeam 1316 of the shutter open actuator 1304 bows away from the driveanchor 1320 of the shutter open actuator 1304. The load beam 1316 of theshutter closed actuator 1306 bows towards the drive anchor 1320 of theshutter close actuator 1306.

In operation, to change states, for example from closed to open, adisplay apparatus applies a potential to the drive beams 1318 of theshutter open actuator 1304. The display apparatus may also apply apotential to the load beams 1316 of the shutter open actuator. Anyelectrical potential difference between the drive beams and the loadbeams, regardless of sign with respect to a ground potential, willgenerate an electrostatic force between the beams. The resultant voltagebetween the drive beams 1318 and the load beams 1316 of the shutter openactuator 1304 results in an electrostatic force, drawing the beams 1316and 1318 together. If the voltage is sufficiently strong, the load beam1316 deforms until its curvature is substantially reversed, as depictedin the shutter close actuator in FIG. 13A.

FIG. 13B shows the evolution of force versus displacement for thegeneral case of bi-stable actuation, including that for FIG. 13A.Referring to FIGS. 13A and 13B, generally the force required to deform acompliant load beam will increase with the amount of displacement.However, in the case of a bi-stable mechanism, such as illustrated inFIG. 13A, a point is reached (point B in FIG. 13B) where further travelleads to a decrease in force. With sufficient voltage applied betweenthe load beam 1316 and the drive beam 1318 of the shutter open actuator1304, a deformation corresponding to point B of FIG. 13B is reached,where further application of force leads to a large and spontaneousdeformation (a “snap through”) and the deformation comes to rest atpoint C in FIG. 13B. Upon removal of a voltage, the mechanism will relaxto a point of stability, or zero force. Point D is such a relaxation orstable point representing the open position. To move the shutter 1302 inthe opposite direction it is first necessary to apply a voltage betweenthe load beam 1316 and the drive beam 1318 of the shutter close actuator1306. Again a point is reached where further forcing results in a largeand spontaneous deformation (point E). Further forcing in the closeddirection results in a deformation represented by point F. Upon removalof the voltage, the mechanism relaxes to its initial and stable closedposition, point A.

In FIG. 13A, the length of the compliant member is longer than thestraight-line distance between the anchor and the attachment point atthe shutter. Constrained by the anchor points, the load beam finds astable shape by adapting a curved shape, two of which shapes constituteconfigurations of local minima in the potential energy. Otherconfigurations of the load beam involve deformations with additionalstrain energy.

For load beams fabricated in silicon, typical as-designed widths areabout 0.2 μm to about 10 μm. Typical as-designed lengths are about 20 μmto about 1000 μm. Typical as-designed beam thicknesses are about 0.2 μmto about 10 μm. The amount by which the load beam is pre-bent istypically greater than three times the as-designed width

The load beams of FIG. 13A can be designed such that one of the twocurved positions is close to a global minimum, i.e. possesses the lowestenergy or relaxed state, typically a state close to zero energy storedas a deformation or stress in the beam. Such a design configuration maybe referred to as “pre-bent,” meaning, among other things, that theshape of the compliant member is patterned into the mask such thatlittle or no deformation is required after release of the shutterassembly from the substrate. The as-designed and curved shape of thecompliant member is close to its stable or relaxed state. Such a relaxedstate holds for one of the two shutter positions, either the open or theclosed position. When switching the shutter assembly into the otherstable state (which can be referred to as a metastable state) somestrain energy will have to be stored in the deformation of the beam; thetwo states will therefore have unequal potential energies; and lesselectrical energy will be required to move the beam from metastable tostable states as compared to the motion from the stable state to themetastable state.

Another design configuration for FIG. 13A, however, can be described asa pre-stressed design. The pre-stressed design provides for two stablestates with equivalent potential energies. This can be achieved forinstance by patterning the compliant member such that upon release ofthe shutter assembly, the compliant member will substantially andspontaneously deform into its stable shape (i.e. the initial state isdesigned to be unstable). Preferably the two stable shapes are similarsuch that the deformation or strain energy stored in the compliantmember of each of those stable states will be similar. The work requiredto move between open and closed shutter positions for a pre-stresseddesign will be similar.

The pre-stress condition of the shutter assembly can be provided by anumber of means. The condition can be imposed post-manufacture by, forinstance, mechanically packaging the substrate to induce a substratecurvature and thus a surface strain in the system. A pre-stressedcondition can also be imposed as a thin film stress imposed by surfacelayers on or around the load beams. These thin film stresses result fromthe particulars of a deposition process. Deposition parameters that canimpart a thin film stress include thin film material composition,deposition rate, and ion bombardment rate during the deposition process.

In FIG. 13A, the load beam is curved in each of its locally stablestates and the load beam is also curved at all points of deformation inbetween the stable states. The compliant member may be comprised,however, of any number of straight or rigid sections of load beam aswill be described in the following figures. In FIG. 18, furthermore,will be shown the design of a bi-stable shutter assembly in whichneither of the two equivalent stable positions possesses, requires, oraccumulates any significant deformation or strain energy. Stress isstored in the system temporarily as it is moved between the stablestates.

FIG. 14 is a top view of the shutter layer 1400 of a second bi-stableshutter assembly. As described above in relation to FIG. 6, reducingresistance to in-plane motion tends to reduce out-of-plane movement ofthe shutter. The shutter layer 1400 is similar to that of the shutterlayer 1300, other than the shutter layer 1400 includes an in-planestiffness-reducing feature, which promotes in-plane movement, and adeformation promoter which promotes proper transition between states. Aswith the shutter layer 1300 of FIG. 13A, the shutter layer 1400 of FIG.14 includes load beams 1402 coupling load anchors 1404 to a shutter1406. To reduce the in-plane stiffness of the shutter assembly and toprovide some axial compliance to the load beams 1402, the load anchors1404 couple to the load beams 1402 via springs 1408. The springs 1408can be formed from flexures, L brackets, or curved portions of the loadbeams 1402.

In addition, the widths of the load beams 1402 vary along their lengths.In particular, the beams are narrower along sections where they meet theload anchors 1404 and the shutter 1406. The points along the load beams1402 at which the load beams 1402 become wider serve as pivot points1410 to confine deformation of the load beams 1402 to the narrowersections 1410.

FIG. 15 is a top view of a shutter layer 1500 of a tri-stable shutterassembly incorporating dual compliant electrode actuators, according toan illustrative embodiment of the invention. The shutter layer 1500includes a shutter open actuator 1502 and a shutter close actuator 1504.Each actuator 1502 and 1504 includes two compliant drive beams 1506physically and electrically coupled to a substrate of a displayapparatus by a drive anchor 1508.

The shutter open actuator 1502, by itself, is an elastic actuator,having one mechanically stable state. Unless otherwise constrained, theshutter open actuator 1502, after actuation would return to its reststate. The shutter open actuator 1502 includes two load beams 1510coupled to load anchors 1512 by L brackets 1514 at one end and to theshutter 1516 via L brackets 1518 at the other end. In the rest state ofthe shutter open actuator 1502, the load beams 1510 are straight. The Lbrackets 1514 and 1518 allow the load beams 1510 to deform towards thedrive beams 1506 of the shutter open actuator 1502 upon actuation of theshutter open actuator 1502 and away from the drive beams 1506 uponactuation of the shutter close actuator 1504.

The shutter close actuator 1504 is similarly inherently elastic. Theshutter close actuator 1504 includes a single load beam 1520 coupled toa load anchor 1522 at one end. When not under stress, i.e., in its reststate, the load beam 1520 is straight. At the opposite end of the loadbeam 1520 of the shutter close actuator 1504, the load beam 1520 iscoupled to a stabilizer 1524 formed from two curved compliant beams 1526connected at their ends and at the center of their lengths. The beams1526 of the stabilizer 1524 have two mechanically stable positions:bowed away from the shutter close actuator 1504 (as depicted) and bowedtowards the shutter close actuator 1504.

In operation, if either the shutter open actuator 1502 or the shutterclose actuator are activated 1504, the load beam 1520 of the shutterclose actuator 1504 is deformed to bow towards the shutter open actuator1504 or towards the drive beams 1528 of the shutter close actuator 1504,respectively, as the shutter 1516 is moved into an actuated position. Ineither case, the length of the shutter close actuator 1504 load beam1520 with respect to the width of the shutter layer 1500 as a whole, isreduced, pulling the beams 1526 of the stabilizer 1524 to bow towardsthe shutter close actuator 1504. After the activated actuator isdeactivated, the energy needed to deform the beams 1526 of thestabilizer 1524 back to its original position is greater than the energystored in the load beams 1510 and 1520 and of the actuators 1502 and1504. Additional energy must be added to the system to return theshutter 1516 to its rest position. Thus, the shutter 1516 in the shutterassembly has three mechanically stable positions, open, half open, andclosed.

FIGS. 16A-C are diagrams of another embodiment of a bi-stable shutterassembly 1600, illustrating the state of the shutter assembly 1600during a change in shutter 1602 position. The shutter assembly 1600includes a shutter 1602 physically supported by a pair of compliantsupport beams 1604. The support beams couple to anchors 1603 as well asto the shutter 1602 by means of rotary joints 1605. These joints may beunderstood to consist of pin joints, flexures or thin connector beams.In the absence of stress being applied to the support beams 1604, thesupport beams 1604 are substantially straight.

FIG. 16A depicts the shutter 1602 in an open position, FIG. 16B depictsthe shutter 1602 in the midst of a transition to the closed position,and FIG. 16C shows the shutter 1602 in a closed position. The shutterassembly 1600 relies upon an electrostatic comb drive for actuation. Thecomb drive is comprised of a rigid open electrode 1608 and a rigidclosed electrode 1610. The shutter 1602 also adopts a comb shape whichis complementary to the shape of the open and closed electrodes. Combdrives such as are shown in FIG. 16 are capable of actuating overreasonably long translational distances, but at a cost of a reducedactuation force. The primary electrical fields between electrodes in acomb drive are aligned generally perpendicular to the direction oftravel, therefore the force of actuation is generally not along thelines of the greatest electrical pressure experienced by the interiorsurfaces of the comb drive.

Unlike the bi-stable shutter assemblies described above, instead ofrelying upon a particular curvature of one or more beams to providemechanical stability, the bi-stable actuator 1600 relies on the straightrelaxed state of its support beams 1604 to provide mechanical stability.For example, in its two mechanically stable positions, depicted in FIGS.16A and 16C, the compliant support beams 1604 are substantially straightat an angle to the linear axis 1606 of the shutter assembly 1600. Asdepicted in FIG. 16B, in which the shutter 1602 is in transition fromone mechanically stable position to the other, the support beams 1604physically deform or buckle to accommodate the movement. The forceneeded to change the position of the shutter 1602 must therefore besufficient to overcome the resultant stress on the compliant supportbeams 1604. Any energy difference between the open and closed states ofshutter assembly 1600 is represented by a small amount of elastic energyin the rotary joints 1605.

The shutter 1602 is coupled to two positions on either side of theshutter 1602 through support beams 1604 to anchors 1603 in positions oneither side of the shutter assembly 1600, thereby reducing any twistingor rotational motion of the shutter 1602 about its central axis. The useof compliant support beams 1604 connected to separate anchors onopposite sides of the shutter 1602 also constrains the movement of theshutter along a linear translational axis. In another implementation, apair of substantially parallel compliant support beams 1604 can becoupled to each side of shutter 1602. Each of the four support beamscouples at independent and opposing points on the shutter 1602. Thisparallelogram approach to support of the shutter 1602 helps to guaranteethat linear translational motion of the shutter is possible.

FIG. 17A depicts a bi-stable shutter assembly 1700 a, in which the beams1702 a incorporated into the shutter assembly 1700 a are substantiallyrigid as opposed to compliant, in both of the shutter assembly's stablepositions 17A-1 and 17A-3 as well as in a transitional position 17A-2.The shutter assembly 1700 a includes a shutter 1704 a driven by a pairof dual compliant beam electrode actuators 1706 a. Two compliant members1710 a support the shutter 1704 a over a surface 1712 a. The compliantmembers 1710 a couple to opposite sides of the shutter 1704 a. The otherends of the compliant members 1710 a couple to anchors 1714 a,connecting the compliant members 1710 a to the surface 1712 a. Eachcompliant member 1710 a includes two substantially rigid beams 1716 acoupled to a flexure or other compliant element 1718 a, such as a springor cantilever arm. Even though the beams 1716 a in the compliant membersare rigid, the incorporation of the compliant element 1718 a allows thecompliant member 1710 a as a whole to change its shape in a compliantfashion to take on two mechanically stable shapes. The compliant elementis allowed to relax to its rest state in either of the closed or openpositions of the shutter assembly (see 17A-1 and 17A-3), so that both ofthe end states possess substantially identical potential energies. Nopermanent beam bending or beam stressing is required to establish thestability of the two end states, although strain energy is stored in thecompliant element 1718 a during the transition between states (see17A-2).

The shape of the compliant element 1718 a is such that a relatively easyin-plane translation of the shutter 1704 a is allowed while out-of-planemotion of the shutter is restricted.

The actuation of the bi-stable shutter assembly 1700 a is accomplishedby a pair of elastic dual compliant beam electrode actuators 1706 a,similar to the actuators employed in FIG. 15. In shutter assembly 1700 athe actuators 1706 a are physically separated and distinct from thecompliant members 1710 a. The compliant members 1710 a provide arelatively rigid support for the shutter 1704 a while providing thebi-stability required to sustain the open and closed states. Theactuators 1706 a provide the driving force necessary to switch theshutter between the open and closed states.

Each actuator 1706 a comprises a compliant load member 1720 a. One endof the compliant load member 1720 a is coupled to the shutter 1704 a,while the other end is free. In shutter assembly 1700 a the compliantload members in actuators 1706 a are not coupled to anchors or otherwiseconnected to the surface 1712 a. The drive beams 1722 a of the actuators1706 a are coupled to anchors 1724 a and thereby connected to thesurface 1712 a. In this fashion the voltage of actuation is reduced.

FIG. 17B is a diagram of a bi-stable shutter assembly 1700 b in whichthe shutter 1702 b is designed to rotate upon actuation. The shutter1702 b is supported at four points along its periphery by 4 compliantsupport beams 1704 b which are coupled to four anchors 1706 b. As inFIG. 16, the compliant support beams 1704 b are substantially straightin their rest state. Upon rotation of the shutter 1702 b the compliantmembers will deform as the distance between the anchors and the shutterperiphery decreases. There are two low energy stable states in which thecompliant support beams 1704 b are substantially straight. The shuttermechanism in 1700 b has the advantage that there is no center of massmotion in the shutter 1702 b.

The shutter 1702 b in shutter assembly 1700 b has a plurality of shutterapertures 1708 b, each of which possesses a segmented shape designed tomake maximum use of the rotational motion of the shutter.

FIG. 18 is a diagram of a bi-stable shutter assembly 1800 incorporatingthermoelectric actuators 1802 and 1804. The shutter assembly 1800includes a shutter 1806 with a set of slotted shutter apertures 1808.Thermoelectric actuators 1802 and 1804 couple to either side of theshutter 1806 for moving the shutter 1806 transversely in a planesubstantially parallel to a surface 1808 over which the shutter 1806 issupported. The coupling of the shutter 1806 from two positions on eitherside of the shutter 1806 to load anchors 1807 in positions on eitherside of the shutter assembly 1800 help reduce any twisting or rotationalmotion of the shutter 1806 about its central axis.

Each thermoelectric actuator 1802 and 1804 includes three compliantbeams 1810, 1812, and 1814. Compliant beams 1810 and 1812 are eachthinner than compliant beam 1814. Each of the beams 1810, 1812, and 1814is curved in an s-like shape, holding the shutter 1806 stably inposition.

In operation, to change the position of the shutter from open (asdepicted) to closed, current is passed through a circuit including beams1810 and 1814. The thinner beams 1810 in each actuator 1802 and 1804heat, and therefore also expands, faster than the thicker beam 1814. Theexpansion forces the beams 1810, 1812, and 1814 from their mechanicallystable curvature, resulting in transverse motion of the shutter 1806 tothe closed position. To open the shutter 1806, current is run through acircuit including beams 1812 and 1814, resulting in a similardisproportionate heating and expansion of beams 1812, resulting in theshutter 1806 being forced back to the open position.

Bi-stable shutter assemblies can be driven using a passive matrix arrayor an active matrix array. FIG. 19 is a diagram of a passive matrixarray 1900 for controlling bi-stable shutter assemblies 1902 to generatean image. As with active matrix arrays, such as active matrix arrays 900and 1000, the passive matrix array 1900 is fabricated as a diffused orthin-film-deposited electrical circuit on a substrate 1904 of a displayapparatus. In general, passive matrix arrays 1900 require less circuitryto implement than active matrix arrays 900 and 1000, and are easier tofabricate. The passive matrix array 1900 divides the shutter assemblies1902 on the substrate 1904 of the display apparatus into rows andcolumns of grid segments 1906 of a grid. Each grid segment 1906 mayinclude one or more bi-stable shutter assemblies 1902. In the displayapparatus, all grid segments 1906 in a given row of the grid share asingle row electrode 1908. Each row electrode 1908 electrically couplesa controllable voltage source, such as driver 1910 to the load anchorsof the shutter assemblies 1902. All shutter assemblies 1902 in a columnshare two common column electrodes, a shutter open electrode 1912 and ashutter close electrode 1914. The shutter open electrode 1912 for agiven column electrically couples a driver 1910 to the drive electrodeof the shutter open actuator of the shutter assemblies 1902 in thecolumn. The shutter close electrode 1914 for a given column electricallycouples a driver 1910 to the drive electrode of the shutter closeactuator of the shutter assemblies 1902 in the column.

The shutter assemblies 1300, 1400, 1500, 1600, 1700 a, and 1800 areamenable to the use of a passive matrix array because their property ofmechanical bi-stability makes it possible to switch between open andclosed states if the voltage across the actuator exceeds a minimumthreshold voltage. If the drivers 1910 are programmed such that none ofthem will output a voltage that by itself is sufficient to switch theshutter assemblies between open and closed states, then a given shutterassembly will be switched if its actuator receives voltages from twoopposing drivers 1910. The shutter assembly at the intersection of aparticular row and column can be switched if it receives voltages fromits particular row and column drivers whose difference exceeds theminimum threshold voltage.

To change the state of a shutter assembly 1902 from a closed state to anopen state, i.e., to open the shutter assembly 1902, a driver 1910applies a potential to the row electrode 1908 corresponding to the rowof the grid in which the shutter assembly 1902 is located. A seconddriver 1910 applies a second potential, in some cases having an oppositepolarity, to the shutter open electrode 1912 corresponding to the columnin the grid in which the shutter assembly 1902 is located. To change thestate of a shutter assembly 1902 from an open state to a closed state,i.e., to close the shutter assembly 1902, a driver 1910 applies apotential to the row electrode 1908 corresponding to the row of thedisplay apparatus in which the shutter assembly 1902 is located. Asecond driver 1910 applies a second potential, in some cases having anopposite polarity, to the shutter close electrode 1914 corresponding tothe column in the display apparatus in which the shutter assembly 1902is located. In one implementation, a shutter assembly 1902 changes statein response to the difference in potential applied to the row electrode1908 and one of the column electrodes 1912 or 1914 exceeding apredetermined switching threshold.

To form an image, in one implementation, a display apparatus sets thestate of the shutter assemblies 1902 in the grid, one row at a time insequential order. For a given row, the display apparatus first closeseach shutter assembly 1902 in the row by applying a potential to thecorresponding row electrodes 1908 and a pulse of potential to all of theshutter close electrodes 1914. Then, the display apparatus opens theshutter assemblies 1902 through which light is to pass by applying apotential to the shutter open electrode 1912 and applying a potential tothe row electrodes 1908 for the rows which include shutter assemblies1902 in the row which are to be opened. In one alternative mode ofoperation, instead of closing each row of shutter assemblies 1902sequentially, after all rows in the display apparatus are set to theproper position to form an image, the display apparatus globally resetsall shutter assemblies 1902 at the same time by applying potentials toall shutter close electrodes 1914 and all row electrodes 1908concurrently. In another alternative mode of operation, the displayapparatus forgoes resetting the shutter assemblies 1902 and only altersthe states of shutter assemblies 1902 that need to change state todisplay a subsequent image. A number of alternate driver control schemesfor images have been proposed for use with ferroelectric liquid crystaldisplays, many of which can be incorporated for use with themechanically bi-stable displays herein. These technologies are describedin Liquid Crystal Displays: Driving Schemes and Electro-Optical Effects,Ernst Lieder (Wiley, New York, 2001).

The physical layout of the display is often a compromise between thecharacteristics of resolution, aperture area, and driving voltage. Smallpixel sizes are generally sought to increase the resolution of thedisplay. As pixels become smaller, however, proportionally the roomavailable for shutter apertures decreases. Designers seek to maximizeaperture ratio as this increases the brightness and power efficiency ofthe display. Additionally, the combination of a small pixels and largeaperture ratios implies large angular deformations in the compliantmembers that support the shutters, which tends to increase the drivevoltages required and the energy dissipated by the switching circuitry.

FIGS. 20A and 20B demonstrate two methods of tiling shutter assembliesinto an array of pixels to maximize the aperture ratios in dense arraysand minimize the drive voltages.

FIG. 20A, for example, depicts a tiling 2000 of two cantilever dual beamelectrode actuator-based shutter assemblies 2002 and 2004 tiled to forma rhombehedral pixel 2006 from two generally triangular shutterassemblies 2002 and 2004. The shutter assemblies 2002 and 2004 may beindependently or collectively controlled. The rhombehedral tiling ofFIG. 20A is quite close to a rectangular tiling arrangement, and in factadapted to a rectangular pixel with aspect ratio of 2:1. Since twoshutter assemblies can be established within each rectangle, such a 2:1rectangular tiling arrangement can further be attached or built on topof an active matrix array which possesses a square repeating distancebetween rows and columns. A 1 to 1 correlation between pixels in the twoarrays can therefore be established. Square pixel arrays are mostcommonly employed for the display of text and graphic images. Theadvantage of the layout in FIG. 20B is that it is understood to maximizethe length of the load beams in each triangular pixel to reduce thevoltage required for switching shutters between open and closed states.

FIG. 20B is an illustrative tiling of a plurality of bi-stable dualcompliant beam electrode-actuator-based shutter assemblies 1300 of FIG.13A. In comparison, for example, to the bi-stable dual compliant beamelectrode-actuator-based shutter assembly 1400 of FIG. 14, the width ofthe shutter 1302 of the shutter assembly 1300 is substantially less thanthe distance between the load anchors 1314 of the shutter assembly 1300.While the narrower shutter 1302 allows for less light to pass througheach shutter assembly 1300, the extra space can be utilized for tighterpacking of shutter assemblies 1300, as depicted in FIG. 20B, withoutloss of length in the load beams. The longer load beams makes itpossible to switch the shutters in the array at reduced voltages. Inparticular, the narrower shutter 1302 enables portions of the actuators1304 and 1306 of the shutter assemblies 1300 to interleave with the gapsbetween actuators 1302 and 1304 of neighboring shutter assemblies 1300.The interleaved arrangement of FIG. 20B can nevertheless still be mappedonto a square arrangement of rows and columns, which is the common pixelconfiguration for textual displays.

The tiling or pixel arrangements for shutter assemblies need not belimited to the constraints of a square array. Dense tiling can also beachieved using rectangular, rhombehedral, or hexagonal arrays of pixels,all of which find applications, for example in video and color imagingdisplays.

FIG. 21 is a cross sectional view of a display apparatus 2100incorporating dual compliant electrode actuator-based shutter assemblies2102, according to an illustrative embodiment of the invention. Theshutter assemblies 2102 are disposed on a glass substrate 2104. Arear-facing reflective layer, reflective film 2106, disposed on thesubstrate 2104 defines a plurality of surface apertures 2108 locatedbeneath the closed positions of the shutters 2110 of the shutterassemblies 2102. The reflective film 2106 reflects light not passingthrough the surface apertures 2108 back towards the rear of the displayapparatus 2100. The reflective aperture layer 2106 can be a fine-grainedmetal film without inclusions formed in thin film fashion by a number ofvapor deposition techniques including sputtering, evaporation, ionplating, laser ablation, or chemical vapor deposition. In anotherimplementation, the rear-facing reflective layer 2106 can be formed froma mirror, such as a dielectric mirror. A dielectric mirror is fabricatedas a stack of dielectric thin films which alternate between materials ofhigh and low refractive index.

The display apparatus 2100 includes an optional diffuser 2112 and/or anoptional brightness enhancing film 2114 which separate the substrate2104 from a backlight 2116. The backlight 2116 is illuminated by one ormore light sources 2118. The light sources 2118 can be, for example, andwithout limitation, incandescent lamps, fluorescent lamps, lasers, orlight emitting diodes. A front-facing reflective film 2120 is disposedbehind the backlight 2116, reflecting light towards the shutterassemblies 2102. Light rays from the backlight that do not pass throughone of the shutter assemblies 2102 will be returned to the backlight andreflected again from the film 2120. In this fashion light that fails toleave the display to form an image on the first pass can be recycled andmade available for transmission through other open apertures in thearray of shutter assemblies 2102. Such light recycling has been shown toincrease the illumination efficiency of the display.

In one implementation the light sources 2118 can include lamps ofdifferent colors, for instance, the colors red, green, and blue. A colorimage can be formed by sequentially illuminating images with lamps ofdifferent colors at a rate sufficient for the human brain to average thedifferent colored images into a single multi-color image. The variouscolor-specific images are formed using the array of shutter assemblies2102. In another implementation, the light source 2118 includes lampshaving more than three different colors. For example, the light source2118 may have red, green, blue and white lamps or red, green, blue, andyellow lamps.

A cover plate 2122 forms the front of the display apparatus 2100. Therear side of the cover plate 2122 can be covered with a black matrix2124 to increase contrast. The cover plate 2122 is supported apredetermined distance away from the shutter assemblies 2102 forming agap 2126. The gap 2126 is maintained by mechanical supports or spacersand/or by an epoxy seal 2128 attaching the cover plate 2122 to thesubstrate 2104. The epoxy 2128 should have a curing temperaturepreferably below about 200° C., it should have a coefficient of thermalexpansion preferably below about 50 ppm per degree C. and should bemoisture resistant. An exemplary epoxy 2128 is EPO-TEK B9021-1, sold byEpoxy Technology, Inc.

The epoxy seal 2128 seals in a working fluid 2130. The working fluid2130 is engineered with viscosities preferably below about 10 centipoiseand with relative dielectric constant preferably above about 2.0, anddielectric breakdown strengths above about 10⁴ V/cm. The working fluid2130 can also serve as a lubricant. Its mechanical and electricalproperties are also effective at reducing the voltage necessary formoving the shutter between open and closed positions. In oneimplementation, the working fluid 2130 preferably has a low refractiveindex, preferably less than about 1.5. In another implementation theworking fluid 2130 has a refractive index that matches that of thesubstrate 2104. In another implementation the working fluid 2130 has arefractive index greater than that of the substrate. In anotherimplementation the working fluid has a refractive index greater than2.0. Suitable working fluids 2130 include, without limitation,de-ionized water, methanol, ethanol, silicone oils, fluorinated siliconeoils, dimethylsiloxane, polydimethylsiloxane, hexamethyldisiloxane, anddiethylbenzene.

In another implementation, the working fluid 2130 is a hydrophobicliquid with a high surface wetting capability. Preferably, its wettingcapabilities are sufficient to wet the front as well as the rearsurfaces of the shutter assemblies 2102. Hydrophobic fluids are capableof displacing water from the surfaces of shutter assemblies 2130. Inanother implementation, the working fluid 2130 contains a suspension ofparticles with diameters in the range of 0.5 to 20 microns. Suchparticles scatter light to increase the viewing angle of a display. Inanother implementation the working fluid 2130 contains dye molecules insolution for absorbing some or all frequencies of visible light toincrease the contrast of the display.

Illustrative methods and materials for forming the reflective apertures2106 on the same substrate as the shutter assemblies 2102 are disclosedin co-owned U.S. patent application Ser. No. 11/361,785, filed Feb. 23,2006, incorporated herein by reference.

A sheet metal or molded plastic assembly bracket 2132 holds the coverplate 2122, shutter assemblies 2102, the substrate 2104, the backlight2116 and the other component parts together around the edges. Theassembly bracket 2132 is fastened with screws or indent tabs to addrigidity to the combined display apparatus 2100. In someimplementations, the light source 2118 is molded in place by an epoxypotting compound.

Display apparatus 2100 is referred to as the MEMS-up configuration,wherein the MEMS based light modulators are formed on a front surface ofsubstrate 2104, i.e. the surface that faces toward the viewer. Theshutter assemblies 2102 are built directly on top of the reflectiveaperture layer 2106. In an alternate embodiment of the invention,referred to as the MEMS-down configuration, the shutter assemblies aredisposed on a substrate separate from the substrate on which thereflective aperture layer is formed. The substrate on which thereflective aperture layer is formed is referred to herein as theaperture plate. For the MEMS-down configuration, the substrate on whichthe MEMS-based light modulators are formed takes the place of coverplate 2122 in display apparatus 2100. In the MEMS-down configuration,the substrate that carries the MEMS-based light modulators is orientedsuch that the MEMS-based light modulators are positioned on the rearsurface of the top substrate, i.e. the surface that faces away from theviewer and toward the back light 2116. The MEMS-based light modulatorsare thereby disposed directly opposite to and across a gap from thereflective aperture layer. Display apparatus corresponding to theMEMS-down configuration are described further in U.S. patent applicationSer. No. 11/361,785, filed Feb. 23, 2006 and U.S. patent applicationSer. No. 11/528,191, filed Sep. 26, 2006, both of which are incorporatedherein by reference.

In various embodiments, it is advantageous for the shutters used inshutter assemblies to overlap the apertures to which they correspond,when the shutters are in the closed position. FIGS. 22A and 22B are topviews of a shutter assembly 2200, similar to the shutter assembly 800 ofFIG. 8, in opened and closed positions, respectively, illustrating suchan overlap. The shutter assembly 2200 includes a shutter 2202 supportedover a reflective aperture layer 2204 by anchors 2206 via portions ofopposing actuators 2208 and 2210. The shutter assembly 2200 is suitablefor inclusion in an array of light modulators included in a displayapparatus.

The shutter 2202 includes three shutter apertures 2212, through whichlight can pass. The remainder of the shutter 2202 obstructs the passageof light. In various embodiments, the side of the shutter 2202 facingthe reflective aperture layer 2204 is coated with a light absorbingmaterial or a reflective material to absorb or reflect, respectively,obstructed light.

The reflective aperture layer 2204 is deposited on a transparentsubstrate, preferably formed from plastic or glass. The reflectiveaperture layer 2204 can be formed from a film of metal deposited on thesubstrate, a dielectric mirror, or other highly reflective material orcombination of materials. The reflective aperture layer 2204 has a setof apertures 2214 formed in it to allow light to pass through theapertures, from the transparent substrate, towards the shutter 2202. Thereflective aperture layer 2204 has one aperture corresponding to eachshutter aperture 2212. For example, for an array of light modulatorsincluding shutter assemblies 2200, the reflective aperture layerincludes three apertures 2214 for each shutter assembly 2200. Eachaperture has at least one edge around its periphery. For example, therectangular apertures 2214 have four edges. In alternativeimplementations in which circular, elliptical, oval, or other curvedapertures are formed in the reflective aperture layer 2204, eachaperture may have only a single edge.

In FIG. 22A, the shutter assembly 2200 is in an open state. Actuator2208 is in an open position, and actuator 2210 is in a collapsedposition. Apertures 2214 are visible through the shutter apertures 2212.As visible, the shutter apertures 2212 are larger in area than theapertures 2214 formed in the reflective aperture layer 2204. The sizedifferential increases the range of angles at which light can passthrough the shutter apertures 2212 towards an intended viewer.

In FIG. 22B, the shutter assembly is a closed state. Actuator 2208 is ina collapsed position and actuator 2210 is in an open position. Lightblocking portions of the shutter 2202 cover the apertures 2214 in thereflective aperture layer 2204. The light blocking portions of theshutter 2202 overlap the edges of the apertures 2214 in the reflectiveaperture layer 2204 by a predefined overlap 2216. In someimplementations, even when a shutter is in a closed state, some light,at angles far from an axis normal to the shutter 2202, may leak throughthe apertures 2214. The overlap included in shutter assembly 2200reduces or eliminates this light leakage. While, as depicted in FIG.22B, the light blocking portions of shutter 2202 overlap all four edgesof the aperture, having the light blocking portions of shutter 2202overlap even one of the edges reduces light leakage.

FIG. 23A-23C are cross-sectional views of various configurations of theshutter assembly 2200 in relation to the transparent substrate on whichthe reflective aperture layer 2204 is formed. The cross sectional viewscorrespond to the line labeled B-B′ on FIGS. 22A and 22B. For purposesof illustration, the shutter 2202 is illustrated in FIGS. 23A-23C ashaving only a single shutter aperture 2323 and two light blockingportions 2324.

FIG. 23A is a cross section of a first configuration of a displayapparatus 2300 including a shutter assembly 2301 similar to thatdepicted in FIG. 22 in the closed state taken across line B-B′,according to an illustrative embodiment of the invention. In the firstconfiguration, the shutter assembly 2301 is formed on a reflectiveaperture layer 2302. The reflective aperture layer 2302 is formed from athin metal film deposited on a transparent substrate 2304. Alternately,the reflective aperture layer 2302 can be formed from a dielectricmirror, or other highly reflective material or combination of materials.The reflective aperture layer 2302 is patterned to form apertures 2306.The transparent substrate 2304 is positioned proximate a light guide2308. The transparent substrate 2304 and the light guide 2308 areseparated by a gap 2309 filled with a fluid, such as air. The refractiveindex of the fluid is preferably less than that of the light guide 2308.Suitable light guides 2308 for display apparatus 2300 are describedfurther in U.S. patent application Ser. No. 11/528,191, the entirety ofwhich is herein incorporated by reference. The display apparatus 2300also includes a front-facing rear reflective layer 2310 positionedadjacent the rear side of the light guide 2308.

The shutter assembly 2301 includes the shutter 2314 supported proximateto the reflective aperture layer 2302 by anchors 2316 via portions ofopposing actuators 2318 and 2320. The anchors 2316 and actuators 2318and 2320 suspend the shutter 2314 at about a constant distance H1(measured from the bottom of the shutter 2314) over the reflectiveaperture layer 2302. In addition, the display apparatus 2300 includes acover plate 2311 supported over the transparent substrate 2304 by spacerposts 2312. The spacer posts 2312 keep the cover plate at about a secondconstant distance H2 away from the top of the shutter 2314. Thesubstrates 2304 and 2311 can be made of a substantially rigid material,such as glass, in which case a relatively low density of spacers 2312may be used to maintain the desired spacing H2. For example, with rigidsubstrates, the display apparatus 2300, in one implementation, includesone spacer 2312 for every 4 pixels, though other densities, both higherand lower, may also be employed. In an alternative implementation eithersubstrate 2304 or 2311 can be made of a flexible material, such asplastic, in which case it is preferable to have a higher density ofspacers 2312, for example, one spacer 2312 within or between each pixelin the array.

The gap between the cover plate 2311 and the transparent substrate 2304is filled with a working fluid 2322, such as working fluid 2130,described above. The working fluid 2322 preferably has a refractiveindex greater than that of the transparent substrate 2304. In anotherimplementation the working fluid has a refractive index greater than2.0. In another implementation the working fluid 2322 has a refractiveindex that is equal to or less than the index of refraction of thetransparent substrate 2304.

As indicated above, the shutter assembly 2301 is in the closed state.Light blocking portions 2324 of the shutter 2314 overlap the edges ofthe apertures 2306 formed in the reflective aperture layer 2302. Thelight blocking properties of shutter 2314 are improved when the gapbetween the shutter and the aperture, i.e. the distance H1, is made assmall as possible. In one implementation, H1 is less than about 100 μm.In another implementation, H1 is less than about 10 μm. In still anotherimplementation, H1 is about 1 μm. In an alternative embodiment thedistance H1 is greater than 0.5 mm, but remains smaller than the displaypitch. The display pitch is defined as the distance between pixels(measured center to center), and in many cases is established as thedistance between apertures, such as apertures 2306, measured center tocenter, in the rear-facing reflective layer 2302.

The size of the overlap W1 is preferably proportional to the distanceH1. While the overlap W1 may be smaller, preferably the overlap W1 isgreater than or equal to the distance H1. In one implementation theoverlap W1 is greater than or equal to 1 micron. In anotherimplementation the overlap W1 is between about 1 micron and 10 microns.In another implementation the overlap W1 is greater than 10 microns. Inone particular implementation, the shutter 2314 is about 4 μm thick. H1is about 2 μm, H2 is about 2 μm, and W1>=2 μm. By having the overlap W1being greater than or equal to H1, if the shutter assembly 2301 is inthe closed state as depicted in FIG. 23A, most light having a sufficientangle to escape the light guide 2308 through the apertures 2306 impactsthe light blocking portions 2324 of the shutter 2314, thereby improvingthe contrast ratio of the display apparatus 2300.

H2 is preferably about the same distance as H1. The spacer posts 2312are preferably formed from a polymer material that is lithographicallypatterned, developed, and/or or etched into cylindrical shapes. Theheight of the spacer is determined by the cured thickness of the polymermaterial. Methods and materials for formation of spacers 2312 aredisclosed in co-owned U.S. patent application Ser. No. 11/361,785, filedFeb. 23, 2006, incorporated herein by reference. In an alternativeembodiment the spacer 2312 can be formed from a metal which iselectrochemically deposited into a mold made from a sacrificialmaterial.

FIG. 23B is a cross section of a second configuration of a displayapparatus 2340 including a shutter assembly 2341 similar to thatdepicted in FIG. 22 in the closed state, according to an illustrativeembodiment of the invention. This second configuration is referred to asthe MEMS-down configuration, in which the reflective aperture layer 2344is formed on a transparent substrate called the aperture plate 2346,which is distinct from the light modulator substrate 2342 to whichshutter assembly 2341 is anchored. The shutter assembly includes ashutter 2354 having light blocking portions 2362 and shutter apertures2363 formed therein. Like the aperture plate 2346, the light modulatorsubstrate 2342 is also transparent. The two substrates 2342 and 2346 areseparated by a gap. The two substrates 2342 and 2346 are aligned duringassembly such that a one to one correspondence exists, as indicated inFIG. 22, between each of the apertures 2347 and the light blockingportions 2362 of shutter 2354 when that shutter is in the closedposition, and/or between the apertures 2347 and the shutter apertures2363 when that shutter is in the open position. In alternativeembodiments, the correspondence between apertures and either lightblocking portions 2362 or shutter apertures 2363 of a shutter 2354 is aone to many or many to one correspondence.

In the MEMS-down display apparatus 2340, the shutter assembly 2341 isformed on the rear-facing surface of the light modulator substrate 2342,i.e. on the side which faces the light guide 2348. In display apparatus2340, the aperture plate 2346 is positioned between the light modulatorsubstrate 2342 and the light guide 2348 The reflective aperture layer2344 is formed from a thin metal film deposited on the front-facingsurface of transparent aperture plate 2346. The reflective aperturelayer 2344 is patterned to form apertures 2347. In anotherimplementation, the reflective layer 2344 can be formed from a mirror,such as a dielectric mirror. A dielectric mirror is fabricated from astack of dielectric thin films with different refractive indices, orfrom combinations of metal layers and dielectric layers.

The aperture plate 2346 is positioned proximate to a backlight or lightguide 2348. The aperture plate 2346 is separated from the light guide2348 by a gap 2349 filled with a fluid, such as air. The refractiveindex of the fluid is preferably less than that of the light guide 2348.Suitable backlights 2348 for display apparatus 2340 are describedfurther in U.S. patent application Ser. No. 11/528,191, the entirety ofwhich is herein incorporated by reference. The display apparatus 2340also includes a front-facing rear reflective layer 2350 positionedadjacent the rear side of the backlight 2348. The front-facingreflective layer 2350 combined with the rear-facing reflective layer2344 forms an optical cavity which promotes recycling of light rayswhich do not initially pass through apertures 2347. The shutter assembly2341 includes the shutter 2354 supported proximate to the transparentsubstrate 2342 by anchors 2356 via portions of opposing actuators 2358and 2360. The anchors 2356 and actuators 2358 and 2360 suspend theshutter 2354 at about a constant distance H4 (measured from the top ofthe shutter 2354) below the light modulator substrate 2342. In addition,display apparatus includes spacer posts 2357, which support the lightmodulator substrate 2342 over the aperture plate 2346. The spacer posts2357 keep the light modulator substrate 2342 at about a second constantdistance H5 away from the aperture plate 2346, thereby keeping thebottom surface of shutter 2354 at a third about constant distance H6above the reflective aperture layer 2344. The spacer posts 2357 areformed in a fashion similar to those of spacers 2312.

The gap between the light modulator substrate 2342 and the apertureplate 2346 is filled with a working fluid 2352, such as working fluid2130, described above. The working fluid 2352 preferably has arefractive index greater than that of the transparent aperture plate2346. In another implementation the working fluid has a refractive indexgreater than 2.0. In another implementation the working fluid 2352preferably has a refractive index that is equal to or less than theindex of refraction of the aperture plate 2346.

As indicated above, the shutter assembly 2341 is in the closed state.Light blocking portions 2362 of the shutter 2354 overlap the edges ofthe apertures 2347 formed in the reflective aperture layer 2344. Thesize of the overlap W2 is preferably proportional to the distance H6.While the overlap W2 may be smaller, preferably the overlap W2 isgreater than or equal to the distance H6. In one implementation, H6 isless than about 100 μm. In another implementation, H6 is less than about10 μm. In still another implementation, H6 is about 1 μm. In analternative embodiment the distance H6 is greater than 0.5 mm, butremains smaller than the display pitch. The display pitch is defined asthe distance between pixels (measured center to center), and in manycases is established as the distance between the centers of apertures inthe rear-facing reflective layer, such as apertures 2347. H4 ispreferably about the same distance as H6. In one particularimplementation, the shutter 2354 is about 4 μm thick, H6 is about 2 μm,H4 is about 2 μm, H5 is about 8 μm and W2>=2 μm. By having the overlapW2 being greater than or equal to H6, if the shutter assembly 2341 is inthe closed state as depicted in FIG. 23B, most light having a sufficientangle to escape the backlight 2348 through the apertures 2347 impactsthe light blocking portions 2362 of the shutter 2354, thereby improvingthe contrast ratio of the display apparatus 2340.

FIG. 23C is a cross section of a third configuration of a displayapparatus 2370 including a shutter assembly 2371 similar to thatdepicted in FIG. 22 in the closed state, according to an illustrativeembodiment of the invention. In comparison to the second configurationof the display apparatus 2340 described above, the display apparatus2370 is designed to account for minor misalignments that may occurduring the aligning and bonding of a light modulator substrate 2372(similar to light modulator substrate 2342) on which a shutter assembly2371 is formed to an aperture plate 2374 (similar to the aperture plate2346) on which a reflective aperture layer 2376 is deposited. To addressthis potential issue, the display apparatus 2370 includes an additionallayer of light absorbing material 2377, deposited on the light modulatorsubstrate 2372. The light absorbing material 2377 may be part of a blackmask, though at least some of the light absorbing material 2377 ispreferably located in the interior of a pixel to which the shutterassembly 2371 corresponds. The light absorbing material 2377 absorbslight 2378 that would otherwise pass through the light modulatorsubstrate 2372 while the shutter 2382 is in the closed state. Additionallight absorbing material 2377 may be deposited on the front side ofreflective aperture layer 2376 to absorb light, for example light 2380deflected from a shutter 2382.

FIG. 23D is a cross section of a fourth configuration of a displayapparatus 2390 including a shutter assembly 2385 similar to thatdepicted in FIG. 22 in the closed state, according to an illustrativeembodiment of the invention. In comparison to the second configurationof shutter assembly 2354 described above, the shutter assembly 2385 isfabricated according to a different process resulting in different crosssectional thicknesses for some of its members. The resulting shutter2393 is referred to herein as a corrugated shutter. The designguidelines for gap distances, e.g. H8 and H10, and for the overlapparameter W4, however, are preferably unchanged from the correspondinggap distances and the overlap parameters described above. The displayapparatus 2390 includes a transparent light modulator substrate 2386,oriented in the MEMS down configuration, and to which the shutterassembly 2385 is attached. The display apparatus 2390 also includes atransparent aperture plate 2387 on which a rear-facing reflectiveaperture layer 2388 is deposited. The display apparatus 2390 includes afluid 2389 which fills the gap between substrates 2386 and 2387. Thefluid 2389 preferably has a refractive index higher than that of theaperture plate 2387. The display apparatus also includes a backlight2348 along with front-facing reflective layer 2350.

The shutter assembly 2385 is in the closed state. Light blockingportions 2391 of the corrugated shutter 2393 overlap the edges ofapertures 2394 formed in the reflective aperture layer 2388. Thecorrugated shutter 2393 is comprised of two connected flat platesections: section 2391 which is oriented horizontally and section 2392which is oriented vertically. Each flat plate 2391 and 2392 is comprisedof thin film materials with thicknesses in the range of 0.2 to 2.0 μm.In a particular embodiment the thickness of the horizontal section 2391is 0.5 μm. The vertical section 2392 provides a stiffness to thecorrugated shutter 2393 and a height which matches that of actuator 2358without requiring the deposition of a bulk materials thicker than about2 am. Methods and materials for formation of shutters with a corrugatedand/or three dimensional structures are disclosed in co-owned U.S.patent application Ser. No. 11/361,785, filed Feb. 23, 2006,incorporated herein by reference.

Similar to dimensions described for display apparatus 2340, in aparticular example the dimensions of H8, H9, and H10 of displayapparatus 2390 can be 2, 8, and 2 μm respectively. The overlap W4 ispreferably greater than or equal to the distance H10. In anotherexample, the distance H10 and the overlap W4 can be >=1 μm. Using thematerials and methods for a corrugated shutter 2393, however, thethickness of section 2391 can be as thin as 0.5 μm. By having theoverlap W4 greater than or equal to H10, if the shutter assembly 2385 isin the closed state as depicted in FIG. 23A, most light having asufficient angle to escape the backlight 2348 through the apertures 2394impacts the light blocking portions 2391 of the shutter 2393, therebyimproving the contrast ratio of the display apparatus 2390.

FIG. 24 is a cross sectional view of a first electrowetting-based lightmodulation array 2400, according to an illustrative embodiment of theinvention. The light modulation array 2400 includes a plurality ofelectrowetting-based light modulation cells 2402 a-2402 d (generally“cells 2402”) formed on an optical cavity 2404. The light modulationarray 2400 also includes a set of color filters 2406 corresponding tothe cells 2402.

Each cell 2402 includes a layer of water (or other transparentconductive or polar fluid) 2408, a layer of light absorbing oil 2410, atransparent electrode 2412 (made, for example, from indium-tin oxide)and an insulating layer 2414 positioned between the layer of lightabsorbing oil 2410 and the transparent electrode 2412. Illustrativeimplementation of such cells are described further in U.S. PatentApplication Publication No. 2005/0104804, published May 19, 2005 andentitled “Display Device,” incorporated herein by reference. In theembodiment described herein, the transparent electrode 2412 takes uponly a portion of a rear surface of a cell 2402.

The remainder of the rear surface of a cell 2402 is formed from areflective aperture layer 2416 that forms the front surface of theoptical cavity 2404. The rear-facing reflective layer 2416 is patternedto form apertures, which in the embodiment of cell 2402 are coincidentwith the transparent electrode 2412. Preferably, when in the closedposition, the layer of light absorbing oil 2410 overlaps one or moreedges of its corresponding aperture in the reflective aperture layer2416. The reflective aperture layer 2416 is formed from a reflectivematerial, such as a reflective metal or a stack of thin films forming adielectric mirror. For each cell 2402, an aperture is formed in thereflective aperture layer 2416 to allow light to pass through. In analternate embodiment, the electrode 2412 for the cell is deposited inthe aperture and over the material forming the reflective aperture layer2416, separated by another dielectric layer.

The remainder of the optical cavity 2404 includes a light guide 2418positioned proximate the reflective aperture layer 2416, and a secondreflective layer 2420 on a side of the light guide 2418 opposite thereflective aperture layer 2416. A series of light redirectors 2421 areformed on the rear surface of the light guide, proximate the secondreflective layer. The light redirectors 2421 may be either diffuse orspecular reflectors. One of more light sources 2422 inject light 2424into the light guide 2418.

In an alternate implementation the light sources 2422 can include lampsof different colors, for instance, the colors red, green, and blue. Acolor image can be formed by sequentially illuminating images with lampsof different colors at a rate sufficient for the human brain to averagethe different colored images into a single multi-color image. Thevarious color-specific images are formed using the array ofelectrowetting modulation cells 2402. In another implementation, thelight source 2422 includes lamps having more than three differentcolors. For example, the light source 2422 may have red, green, blue andwhite lamps or red, green, blue, and yellow lamps.

In an alternative implementation, the cells 2402 and the reflectiveaperture layer 2416 are formed on an additional light modulatorsubstrate which is distinct from light guide 2418 and separated from itby a gap. (See for example the light modulator substrate 2513 of FIG.25.) In yet another implementation, a layer of material with arefractive index less than that of the light guide 2418 is interposedbetween the reflective aperture layer 2416 and the light guide 2418. Thelayer of material with lower refractive index may help to improve theuniformity of light emitted from the light guide 2418.

In operation, application of a voltage to the electrode 2412 of a cell(for example, cell 2402 b or 2402 c) causes the light absorbing oil 2410in the cell to collect in one portion of the cell 2402. As a result, thelight absorbing oil 2410 no longer obstructs the passage of lightthrough the aperture formed in the reflective aperture layer 2416 (see,for example, cells 2402 b and 2402 c). Light escaping the backlight atthe aperture is then able to escape through the cell and through acorresponding color (for example, red, green, or blue) filter in the setof color filters 2406 to form a color pixel in an image. When theelectrode 2412 is grounded, the light absorbing oil 2410 covers theaperture in the reflective aperture layer 2416, absorbing any light 2424attempting to pass through it (see for example cell 2402 a).

The area under which oil 2410 collects when a voltage is applied to thecell 2402 constitutes wasted space in relation to forming an image. Thisarea cannot pass light through, whether a voltage is applied or not, andtherefore, without the inclusion of the reflective portions ofreflective apertures layer 2416, would absorb light that otherwise couldbe used to contribute to the formation of an image. However, with theinclusion of the reflective aperture layer 2416, this light, whichotherwise would have been absorbed, is reflected back into the lightguide 2420 for future escape through a different aperture.

FIG. 25 is a cross sectional view of a second electrowetting-based lightmodulation array 2500, according to an illustrative embodiment of theinvention. The second electrowetting-based light modulation array 2500includes three sub-arrays 2501 a, 2501 b, and 2501 c of coloredelectrowetting-based light modulation cells 2502 (generally “cells2502”), positioned on top of one another. Each cell 2502 includes atransparent electrode 2504, and a colored oil 2506 separated by aninsulator 2508. In one implementation, the oil 2506 in the cells 2502 ofsub-array 2501 a is colored cyan, the oil 2506 in the cells 2502 ofsub-array 2501 b is colored yellow, and the oil 2506 in the cells 2502of sub-array 2501 c is colored magenta. The cells 2502 in sub-array 2501a and the cells 2502 of sub-array 2501 b share a common layer of water2520. The cells 2502 of sub-array 2501 c include their own layer ofwater 2520.

The electrowetting-based light modulation array 2500 includes alight-recycling optical cavity 2510 coupled to the three sub-arrays 2501a-2501 c. The optical cavity 2510 includes a light guide 2512 and alight modulator substrate 2513, separated from the light guide 2512 by agap 2515. The front surface of the light modulator substrate 2513includes a rear-facing reflective aperture layer 2514. The reflectiveaperture layer 2514 is formed from a layer of metal or a stack of thinfilms forming a dielectric mirror. Apertures 2516 are patterned into thereflective aperture layer beneath the cells 2502 of the sub-arrays 2501a-2501 c to allow light to escape the light guide and pass through thesub-arrays 2501 a-2501 c to form an image. The transparent electrodes2504 of cells 2502 are formed over the top of the reflective aperturelayer 2514.

The substrates, i.e., light guide 2512 and modulator substrate 2513, areseparated by a gap 2515 filled with a fluid, such as air. The refractiveindex of the fluid is less than that of the light guide 2512. Afront-facing reflective layer 2518 is formed on, or positioned proximateto, the opposite side of the light guide 2512. The light modulationarray 2500 includes at least one light source 2522 for injecting lightinto the light guide 2512. Suitable light guides 2618 for displayapparatus 2600 are described further in U.S. patent application Ser. No.11/528,191, the entirety of which is herein incorporated by reference.

FIG. 26 is a cross sectional view of a third electrowetting-based lightmodulation array 2600, according to an illustrative embodiment of theinvention. The light modulation array 2600 includes a plurality ofelectrowetting-based light modulation cells 2602 a-2602 c (generally“cells 2602”) formed on an optical cavity 2604. The light modulationarray 2600 also includes a set of color filters 2606 corresponding tothe cells 2602.

While the array 2400 might be considered an example of an array in aMEMS-up configuration, the array 2600 is an example of anelectrowetting-based array assembled in a MEMS-down configuration. Eachcell 2602 includes a layer of water (or other transparent conductive orpolar fluid) 2608, a layer of light absorbing oil 2610, a transparentelectrode 2612 (made, for example, from indium-tin oxide) and aninsulating layer 2614 positioned between the layer of light absorbingoil 2610 and the transparent electrode 2612. In the MEMS-downconfiguration of light modulator array 2600, however, both theinsulating layer 2614 and the transparent electrode 2612 are disposed ona light modulator substrate 2630 distinct from an aperture plate 2632.Like the light modulator substrate 2630, the aperture plate 2632 is alsoa transparent substrate. The light modulator substrate 2630 is thetopmost substrate and is oriented such that control electrodes such astransparent electrode 2612 are disposed on the rear surface of substrate2630, i.e. the surface that faces away from the viewer and toward thelight guide. In addition to transparent electrode 2612, the rear surfaceof light modulator substrate 2630 can carry other common components of aswitching or control matrix for the modulator array, including withoutlimitation, row electrodes, column electrodes, transistors for eachpixel and capacitors for each pixel. The electrodes and switchingcomponents formed on light modulator substrate 2630, which govern theactuation of light modulators in the array, are disposed opposite to andacross a gap 2636 from a reflective aperture layer 2616, disposed on thefront surface of aperture plate 2632. The gap 2636 is filled with theelectrowetting fluid components water 2608 and oil 2610.

The reflective aperture layer 2616 is deposited on transparent substrate2632, preferably formed from plastic or glass. The reflective aperturelayer 2616 can be formed from a film of metal deposited on thesubstrate, a dielectric mirror, or other highly reflective material orcombination of materials. The reflective aperture layer 2616 is arear-facing reflective layer, forming the front surface of opticalcavity 2604. The reflective aperture layer 2616 has a set of apertures2617 formed in it to allow light to pass through the apertures towardthe electrowetting fluid components 2608 and 2610. Optionally, theaperture plate 2632 includes a set of color filters 2606 deposited onthe top surface of reflective aperture 2616 and filling the apertures2617.

The aperture plate 2632 is positioned between the light modulatorsubstrate 2630 and the light guide 2618. The substrates 2632 and 2618are separated from each other by a gap 2634 filled with a fluid (such asair). The refractive index of the fluid is less than that of the lightguide 2618. Suitable light guides 2618 for display apparatus 2600 aredescribed further in U.S. patent application Ser. No. 11/528,191, theentirety of which is herein incorporated by reference. The opticalcavity 2604 also includes substrates 2632, 2618, and the front-facingrear reflective layer 2620 positioned adjacent the rear side of thelight guide 2618. One or more light sources 2622 inject light into thelight guide 2618.

The reflective aperture layer 2616 has one aperture 2617 correspondingto each light modulator cell 2602 in the array 2600. Similarly, thelight modulator substrate 2630 has one transparent electrode 2612 or oneset of pixel transistors and capacitors for each light modulator cell2602. The substrates 2630 and 2632 are aligned during assembly to ensurethat corresponding apertures 2617 are positioned where light will not beobstructed by the oil 2610 when cells are actuated or held in the openstate, e.g. cell 2602 b.

Fabrication of an Aperture Plate

The aperture plate 2700 of FIG. 27 illustrates the detailed structureswithin one implementation of an aperture plate, such as aperture plate2346, 2374, 2387, or 2632 according to an illustrative embodiment of theinvention. The aperture plate 2700 includes a substrate 2702, adielectrically enhanced metal mirror 2704, a light absorbing layer 2706,and a spacer post 2708. The dielectrically enhanced metal mirror and thelight absorbing layer have been patterned into apertures 2709.

The substrate 2702 is preferably a transparent material, for exampleglass or plastic. The dielectrically enhanced metal mirror 2704 iscomprised of a 5-layer stack of materials including, in order from thesubstrate up, a thin film of Si₃N₄ 2710, a thin film of SiO₂ 2712,another thin film of Si₃N₄ 2710, another thin film of SiO₂, 2712, and athin film of aluminum 2714. The relative thicknesses and preferredrefractive indices of these layers are given in Table 1:

TABLE 1 Film Thicknesses and Refractive Indices for a DielectricallyEnhanced Metal Mirror Thin film material Thickness Refractive index 5.Aluminum 200 nm or less NA 4. SiO₂  88 nm 1.46 3. Si₃N₄  64 nm 2.0 2.SiO₂  88 nm 1.46 1. Si₃N₄  64 nm 2.0

The light absorbing layer 2706 can be formed from a thin film of blackchrome, which is a composite of chromium metal particles suspended in anoxide or nitride matrix. Examples include Cr particles in a Cr₂O₃ matrixor Cr particles in an SiO₂ matrix. In other implementations black chromecan be formed from a thin metal film of chromium upon which a thin filmof CrOx (a sub-oxide of chromium) has been either grown or deposited. Apreferred thickness for the black chrome is 150 nm.

The aperture windows 2709 can be patterned from the thin film stack ofmaterials 2704 and 2706 by processes known in the art such asphotolithography and etch or by photolithography and lift-off. In theetch process a layer of photoresist is added to the top of the thin filmstack and then exposed to UV light through a mask. After developing theaperture pattern in the exposed layer of photoresist, the whole stack isetched in the region of apertures 2709 down to the substrate 2702. Suchetching may be accomplished by immersion in wet chemicals, by a dryplasma or ion beam etch, or any combination of the above. In thelift-off process the layer of photoresist is added to the glass beforedeposition of the thin film stack, the resist being developed into apattern that is a reverse of the etch mask pattern. The thin film stackis then deposited over the top of the photoresist, such that the thinfilm stack makes contact to the glass everywhere except in the regionsof the apertures 2709. After deposition of the thin film stack iscomplete, the substrate is dipped into a bath of chemicals thatdissolves or lifts-off the photoresist as well as any thin filmmaterials that were deposited on top of the photoresist.

The spacer post 2708 is formed from a photo-imagable polymer such assuch as a photo-imagable epoxy (in particular a novolac epoxy) or aphoto-imagable polyimide material. Other polymer families that can beprepared in photo-imagable form and are useful for this applicationinclude polyarylene, parylene, benzocyclobutane, perfluorocyclobutane,silsequioxane, and silicone polymers. A particular photo-imagable resistuseful for the spacer application is the Nano SU-8 material availablefrom Microchem Corporation, headquartered in Newton, Mass.

The polymer spacer material is initially deposited as a thick film ontop of the thin film stack 2704 and 2706 after the apertures 2709 havebeen patterned. The photo-imagable polymer is then exposed to UV lightthrough a mask. Alignment marks help to ensure that the resultantspacers 2708 are located correctly with respect to apertures 2709. Forinstance, alignment fiducials can be formed on the periphery of thedisplay during the process of etching the apertures 2709. Thesefiducials are then aligned to a corresponding set of fiducials on theexposure mask to ensure a correct location of spacers 2708. A developingprocess is then effective at removing all of the polymer except where itwas exposed to the UV light. In an alternate method, the features on theexposure mask may be aligned directly to display features on thesubstrate 2702, such as the apertures 2709.

In the particular implementation described with respect to displayapparatus 2340, the spacer posts can be 8 microns tall. In otherimplementations spacer heights may range from about 2 microns to about50 microns. When cross sectioned in the plane of the substrate 2702, thespacers may take regular shapes such as a cylinder or a rectangle withwidths in the range of 2 to 50 microns. Alternately, they can havecomplex irregular cross sections which are designed to maximize thecontact area of the spacer while fitting between other structures on thesubstrate, such as apertures 2709. In a preferred implementation thespacer size, shape and placement is determined so that the spacers donot interfere with the movement of the shutters, such as shutters 2354or other MEMS components, such as actuators 2358 in display apparatus2340.

In another embodiment, the spacer post 2708 is not provided as a polymermaterial but is instead composed of a heat re-flowable joining material,such as a solder alloy. Exemplary heat re-flowable materials aredescribed below with respect to FIG. 29B. The solder alloy can passthrough a melting or re-flow step which allows the solder alloy to wetor bond to a mating surface on the opposing substrate. The solder alloytherefore performs an additional function as a joining material betweenan aperture plate, such as aperture plate 2346 and a modulatorsubstrate, such as substrate 2342. Because of the reflow process, thesolder alloy typically relaxes to an oblate shape referred to as thesolder bump. A predetermined spacing between substrates can bemaintained through control over the average volume of material in thesolder bump. Solder bumps can be applied to aperture plate 2700 by meansof thin film deposition, by thick film deposition through a stencilmask, or by electroplating.

In another embodiment, the aperture plate 2700 can be subjected to asandblasting treatment after the steps of forming the optical layers2704 and 2708. The sandblasting has the effect of roughening thesubstrate surface selectively in the regions of the aperture 2709. Aroughened surface at aperture 2709 behaves as an optical diffuser whichcan provide the benefits of a wider viewing angle for the display. Inanother embodiment, a diffusing surface at aperture 2709 is provided bymeans of an etching process, where the etch is selectively applied inthe regions of apertures 2709 after exposure of photoresist to aphotomask. Etch pits or trenches can be created through proper design ofthe photomask, and the sidewall angles or depths of the pits or trenchescan be controlled by means of either a wet or dry etch process. In thisfashion optical structures with controlled degrees of diffusivebroadening can be created. In this fashion anisotropic diffusers can becreated at the substrate surface which deflect light along a preferredoptical axis, creating elliptical and/or multi-directional cones ofemitted light.

In another embodiment, an etched trench can be provided in substrate2702 that substantially surrounds the display along the periphery of thearray of apertures 2709 (i.e. around the periphery of the active displayregion). The etched trench performs as a mechanical locating structurefor restricting the motion or flowing of an adhesive, such as adhesive2128, used to seal aperture plate 2700 to an opposing substrate.

Further details regarding the materials and processes described abovecan be found in U.S. patent application Ser. No. 11/361,785, filed Feb.23, 2006, incorporated herein by reference. For example, thatapplication includes additional materials and processing methodologiesregarding the formation of dielectrically enhanced metal mirrors withapertures, light absorbing layers, and spacer posts. Although dielectricmirrors and spacers are described in that application in the context ofan integrated (for example MEMS-up) display design, it will beunderstood that similar processes can be adapted to the fabrication ofan aperture plate, such as aperture plate 2700.

In some implementations of the aperture plate 2700, it is desirable toemploy a transparent plastic material for the substrate 2702. Applicableplastics include, without limitation, polymethylmethacrylate (PMMA) andpolycarbonate. When plastic materials are used, it also becomes possibleto utilize an injection molding or stamping process for the formation ofspacer posts 2708. In such a process the spacer posts are formed in amold or a stamper first, before the application of the dielectricallyenhanced metal mirror 2704. All of the layers of the dielectricallyenhanced metal mirror 2704 would be then be deposited in sequence on topof the substrate which already includes spacer posts 2708. The lightabsorbing layer 2706 is deposited on top of the dielectric mirror 2704.In order to pattern the aperture window 2709 a special photoresist isapplied that uniformly coats the surfaces of the thin films withoutbeing disrupted by the presence of spacer posts 2708. Suitablephotoresists include spray-on photoresists and electroplatedphotoresists. Alternately, a spin-on resist is applied followed by areflow step that provides an even resist thickness across the thin filmsurfaces in the areas of apertures 2709. The exposure of the resist,developing, and etching of the thin film layers then proceeds asdescribed above. After the removal of the photoresist, the process iscomplete. A liftoff process can also be employed to pattern thedielectrically enhanced mirror as described above. The use of a moldingor stamping process for the formation of spacer posts 2708 helps toreduce the material costs required in the fabrication of aperture plate2700:

In some display implementations, aperture plate 2346 is combined withlight guide 2348 into one solid body, referred to herein as a unitary orcomposite backlight, described further in U.S. patent application Ser.Nos. 11/218,690 and 11/528,191, respectively. Both applications areincorporated herein by reference. All of the processes described abovefor the formation of the dielectrically enhanced metal mirror 2704, forthe light absorbing layer 2706, and/or for the spacer posts 2708 can besimilarly applied to a substrate which is bonded to or otherwiseindistinguishable from the light guide. The surface of the unitarybacklight onto which the thin films are applied can be glass, or itcould be plastic, including a plastic which has been molded to formspacer posts, such as spacers post 2357.

In one implementation, the spacer posts 2708 are formed or attached toaperture plate 2700 before the aperture plate is aligned to a modulatorsubstrate, such as modulator substrate 2342. In an alternativeimplementation of display apparatus 2340, the spacer posts 2357 arefabricated on top of and as a part of the modulator substrate 2342,before the modulator substrate is aligned to the aperture plate 2346.Such an implementation was described with respect to FIG. 20 within theaforementioned U.S. patent application Ser. No. 11/361,785.

FIG. 28 is a cross sectional view of a display according to anillustrative embodiment of the invention. The display assembly 2800comprises a modulator substrate 2802 and an aperture plate 2804. Thedisplay assembly 2800 also includes a set of shutter assemblies 2806 anda reflective aperture layer 2808. The reflective aperture layer 2805includes apertures 2810. A predetermined gap or separation H12 betweenthe substrate 2802 and 2804 is maintained by the opposing set of spacers2812 and 2814. The spacers 2812 are formed on or as part of themodulator substrate 2802. The spacers 2814 are formed on or as part ofthe aperture plate 2804. During assembly, the two substrates 2802 and2804 are aligned so that spacers 2812 on the modulator substrate 2802make contact with their respective spacers 2814.

The separation or distance H12 of this illustrative example, is 8microns. To establish this separation, the spacers 2812 are 2 micronstall and the spacers 2814 are 6 microns tall. Alternately, both spacers2812 and 2814 can be 4 microns tall, or the spacers 2812 can be 6microns tall while the spacers 2814 are 2 microns tall. In fact, anycombination of spacer heights can be employed as long as their totalheight establishes the desired separation H12.

Providing spacers on both of the substrates 2802 and 2804, which arethen aligned or mated during assembly, has advantages with respect tomaterials and processing costs. The provision of a very tall (e.g. 8micron) spacer, such as spacer 2708, can be costly as it can requirerelatively long times for the cure, exposure, and development of aphoto-imagable polymer. The use of mating spacers as in display assembly2800 allows for the use of thinner coatings of the polymer on each ofthe substrates.

In another implementation, the spacers 2812 which are formed on themodulator substrate 2802 can be formed from the same materials andpatterning steps that were used to form the shutter assemblies 2806. Forinstance, the anchors employed for shutter assemblies 2806 (similar toanchors 2356) can also perform a function similar to spacer 2812. Inthis implementation a separate application of a polymer material to forma spacer would not be required and a separate exposure mask for thespacers would not be required.

The display assembly 2900 of FIG. 29A illustrates one methodology foraligning a modulator substrate and an aperture plate, according to anillustrative embodiment of the invention. The display assembly 2900comprises modulator substrate 2902 and aperture plate 2904. The displayassembly 2900 also comprises a set of shutter assemblies 2906 and areflective aperture layer 2908, including apertures 2910. Apredetermined gap or separation between the substrate 2902 and 2904 ismaintained by the spacers 2912. The spacers 2912 are formed on or aspart of the aperture plate 2904. The display assembly 2900 also includesa set of alignment guides 2914. These alignment guides are formed on oras part of the modulator substrate 2902. When the modulator substrate2902 and aperture plate 2904 are assembled together, the gap between thealignment guides 2914 and the spacers 2912 is quite close, in some casesless than 1 micron. By capturing the spacer posts between the closelyspaced alignment guides, sideways motion between the modulator substrate2902 and the aperture plate 2904 is restricted, thereby maintaining thealignment between the shutters 2906 and the aperture 2910.

In various implementations, the alignment guides 2914 are ring ordoughnut-shaped. In other implementations, the alignment guide 2914 is asimple slot which captures a wall like shape from the aperture plate2904. The alignment slots can be oriented parallel to either or bothedges of the modulator substrate 2903. Having alignment slots withdifferent, and preferably perpendicular, orientations helps to preventmotion in any direction parallel to the plane of the substrate 2902,though other orientations may also be employed The alignment guides 2914can be placed either between pixels, within pixels, or external to thearray of pixels along the periphery of the display.

Alternate means of maintaining alignment between modulator substratesand aperture plates are possible. In one implementation an adhesive,such as adhesive 2128 in FIG. 21, is provided for holding two substratestogether in lateral alignment. In this implementation an alignmentdevice, for example, a mechanical platform equipped with translationalmotor drives and an alignment camera, is utilized to hold the twosubstrates in their proper orientation while an adhesive, such asadhesive 2128, is dried or cured in place. Epoxies that are partially ortotally cured by means of UV radiation are particularly useful asadhesives in this implementation. In implementations where the adhesiveis applied at the periphery of the display assembly, it is referred toas an edge seal or gasket seal. In some implementations the edge sealadhesive contains glass or polymer beads which act as spacers formaintaining a predetermined gap or spacing between the opposingsubstrates.

The display assembly 2950 of FIG. 29B illustrates another means foraligning a modulator substrate to an aperture plate, according to anillustrative embodiment of the invention. The display assembly 2950comprises modulator substrate 2952 and aperture plate 2954. The displayassembly 2950 also comprises a set of shutter assemblies 2956 and areflective aperture layer 2958, including apertures 2960. Apredetermined gap or separation between the substrate 2952 and 2954 ismaintained by the opposing set of spacers 2962 and 2964. The spacers2962 are formed on or as part of the modulator substrate 2952. Thespacers 2964 are formed on or as part of the aperture plate 2954.

The spacers 2962 and 2964 are made of different materials. For theembodiment of display assembly 2950 the spacers 2962 are made of a heatre-flowable material such as solder while the spacers 2964 are made of amaterial which is substantially solid or has a melting or softeningpoint considerably higher than that of spacer 2962. The material in usefor substantially solid spacer 2964 can be any of the materialsdescribed above with respect to spacer post 2708, including thematerials listed in the incorporated U.S. patent application Ser. No.11/361,785, or it can be any of the materials described below withrespect to conductive spacer 3112.

The spacer 2962, also referred to as a solder bump, can be made of anumber of different metals or metal alloys commonly used for thesoldering of electrical connections. Exemplary alloys include, withoutexclusion, Pb—Sn alloys, Pb—In alloys, In—Sn alloys, In—Cu—Sn alloys,Au—Sn alloys, Bi—Sn alloys, or the substantially pure metals In, Sn, Gaor Bi. Such alloys are designed to liquefy or re-flow at temperatures inthe range of 150 to 400 Centigrade and to wet the surfaces of twoopposing contact materials. After cooling and solidification the soldermaterials join together (and optionally electrically connect) the twoopposing contact materials, acting as an adhesive. For the applicationillustrated by display assembly 2950 the solder material 2962 acts as anadhesive link between spacer post 2964 and the modulator substrate 2952.Non-metallic reflow materials are also applicable for use as spacer2962. These materials include glass frit materials, such as mixtures ofbarium-silicate or lead-silicate glasses or thermoplastic polymers, suchas polyethylene, polystyrene, or polypropylene, and/or natural andsynthetic waxes such as carnauba wax, paraffin, or olefin waxes.

The assembly process for display 2950 would proceed as follows. Firstthe spacer materials 2962 and 2964 would be fabricated onto theirrespective substrates. Next the two substrates 2952 and 2954 would beassembled together with roughly the correct lateral alignment. Next thetwo substrates would be heated so as to liquefy or reflow the soldermaterial 2962. Once molten, the material 2962 would proceed to wet thesurface of the substantially solid and opposing spacer post 2964.Simultaneously, the surface tension of the now liquid material 2962 willact to minimize its surface area. The resulting capillary forces havethe effect of pulling or sliding the two substrates 2952 and 2954laterally into a more perfect alignment. After cooling andsolidification the two substrates are locked into alignment by theadhesive properties of the solder material 2962.

In an alternate implementation the substantially solid material can befabricated onto the modulator substrate while the heat re-flowablematerial is fabricated onto the aperture plate. In anotherimplementation both the solid and reflow materials are formedsequentially onto one or the other of the modulator substrate or theaperture plate. For that implementation the solder material is designedto wet and join to a bonding pad located on the opposing substrate.

There are other bonding geometries where a heat re-flowable material canbe used to ensure the alignment between opposing substrates. In oneimplementation solder bumps are fabricated on both of the substrates,and the bumps or beads of solder material are merged or joined duringthe reflow process. In this implementation a substantially solid spacermaterial, such as spacer 2964 is not employed. The gap between the twosubstrates is determined by the average volume of the solder bumps aftersolidification, which can be controlled by means of the fabricateddimensions for the solder bumps prior to assembly. In anotherimplementation the spacer post 2964 is replaced with a detent or solderreceptacle structure, such as a ring or a square frame formed on one ofthe two substrates with an indent at the center. The molten solder tendsto wet and fill the gap at the center of the ring, thereby pulling theopposing substrates into alignment. Similar processes related to thealignment of semi-conductor packages are described in further detail inU.S. Pat. No. 5,477,086, the entirety of which is incorporated herein byreference.

In some implementations the heat re-flowable material is disposed withinor between each of the pixels in the array. In other implementations thepairing of re-flow materials to corresponding posts, receptacles, orbond pads on the opposing substrate can be arranged at the periphery ofthe display or at the outside corners of the display assembly. When there-flow material is disposed at the periphery of the display it can, insome implementations, serve a further purpose similar to epoxy 2128 (seeFIG. 21) as a sealing or gasket material.

FIGS. 30A and 30B are top views of a display assembly 3000, similar tothe shutter assembly 2200 of FIGS. 22A and 22B, in opened and closedpositions, respectively, according to an illustrative embodiment of theinvention. The display assembly 3000 includes a shutter assembly,including shutter 3002, supported over a reflective aperture layer 3004.The shutter assembly also includes portions of opposing actuators 3008and 3010. In display assembly 3000 the shutter is connected to themodulator substrate (not shown), such as modulator substrate 2342 viathe actuators 3008 and 3010 and the anchors 3006 and 3007. Thereflective aperture layer 3004 is formed on a separate substrate, suchas aperture plate 2346. The shutter assembly 3001 is suitable forinclusion in an array of light modulators included in a displayapparatus.

The shutter 3002 includes three shutter apertures 3012, through whichlight can pass. The remainder of the shutter 3002 obstructs the passageof light. The reflective aperture layer 3004 includes apertures 3014which also allow the passage of light. In FIG. 30A, where the shutter isin the open position, the apertures 3012 and 3014 are aligned to allowpassage of light. In FIG. 30B, where the shutter is in the closedposition, the shutter 3002 obstructs the passage of light throughapertures 3014.

The display apparatus 3000 includes spacer posts 3020 which function ina manner similar to spacer posts 2357 of display apparatus 2340 formaintaining a predetermined gap or spacing between opposing substrates.The spacer posts 3020, however, provide an additional function ofensuring the proper alignment between the shutter 3002 and the apertures3014 by fitting between motion stops 3022, which are solid extensions ofthe shutter 3002. When in the open position, the shutter 3002, via oneset of motion stops, comes into hard contact with the spacer post 3020.When in the closed position the shutter 3002, via another set of motionstops 3022, contacts the spacer post 3020 again. The amount of motionallowed for the shutter 3002 between each of the stop positions, invarious implementations, ranges from about 5 to about 50 microns.

The alignment control methodology of display assembly 3000 is effectivefor the case where both apertures 3014 and spacer posts 3020 areattached to one substrate, e.g. the aperture plate, while the shutter3002 is attached by means of anchors 3007 to another substrate, e.g. themodulator substrate. In such implementations, despite misalignments ofas much as 5 or 10 microns during assembly of the two substrates, aproper alignment between the shutters 3002 and the apertures 3014 can beprovided. Thus, a shutter/aperture overlap, e.g. overlap W1 in displayapparatus 2300, of as narrow as 1 micron can be maintained.

FIG. 31 is a cross sectional view of a display assembly 3100, accordingto another illustrative embodiment of the invention. The displayassembly 3100 includes a modulator substrate 3102 and an aperture plate3104. The display assembly 3100 also includes a set of shutterassemblies 3106 and a reflective aperture layer 3108. Additionally, theaperture plate 3104 includes a dielectric isolation layer 3114 and anelectrically conductive interconnect 3116. The cross sectional view inFIG. 31 is taken along a line where these are no aperture holes in thereflective layer 3108. A predetermined gap or separation between thesubstrate 3102 and 3104 is maintained by a set of spacers 3112.

The spacers 3112 of display assembly 3100 include an electricallyconductive material. For example, they can be formed from metal poststhat are formed by either an electroplating process, an etching process,or a lift-off process. The metals copper, nickel, aluminum, gold, ortitanium, without exclusion, are useful for this application.Alternately the posts 3112 can be formed from a composite material, forexample, a polymer or epoxy material that is impregnated with metalparticles to render it conductive. Alternatively the posts 3112 can beformed from polymer materials that are coated with thin metal films suchthat the surface is made conductive.

The electrically conductive spacers 3112 are disposed to provide anelectrical contact between one electrode of the shutter assemblies 3106and the electrically conductive interconnect 3116. For instance, asshown in FIG. 31, the interconnect 3116 can be patterned as a metal linethat is parallel to one of the row or columns in the display. Thespacers 3112 and the interconnect 3116 provide, then, a commonelectrical connection between electrodes in all of the shutterassemblies 3106 for the given row or column. In this fashion, some ofthe metal layers used to form an addressing or control matrix for adisplay, such as display apparatus 2340, can be fabricated on top of theaperture plate 3104 instead of on the modulator substrate 3102. Thespacers 3112 can be configured to provide a distinct electricalconnection between the light modulators 3106 within each of the pixelsin the array to an electrical circuit on the opposing substrate. In anextreme case each of the electrodes for each of the shutter assembliescan be connected to a circuit on the opposing substrate by means ofelectrically conductive spacer posts, such that the complete controlcircuit, including transistors and capacitors, can be formed on asubstrate that is distinct from or separate from that of the lightmodulators.

The electrically conductive spacer posts 3112 can be formed onto eitherthe modulator substrate 3102 or the aperture plate 3104 prior toassembly. In one implementation the conductive spacers are formed fromgold-alloy studs that are individually placed and bonded by machine ontoone or the other of substrates 3102 or 3104 as part of the assemblyprocess. In another implementation the spacers are formed from solderbumps which are either electroplated or stenciled onto one or the otheror both of substrates 3102 or 3104 as part of the assembly process. Anyof the solder materials listed above with respect to solder bump 2962can be employed for spacer posts 3112.

The process of matching or making electrical connection between a spacerposts and a landing zone on the opposing substrate can include a reflowprocess, such as an interdiffusion or a soldering process, so that agood electrical contact is made between the two surfaces.

Although the display assembly 3100 is illustrated as part of a MEMS-downconfiguration, an analogous use for electrically conductive spacer postscan also be found in the MEMS-up configuration as with, for example, thedisplay apparatus 2300. For a MEMS-up configuration both the lightmodulators and the reflective aperture layer are formed on the samesubstrate, while the electrically conductive interconnects, such asinterconnects 3116 are formed on an opposing substrate, such as a coverplate. Conductive spacers, such as spacers 3112, would then makeelectrical connection between the modulator substrate and the coverplate. The invention is particularly useful for applications where thecover plate also acts as a touch-screen input device. Electricalconnections between substrates, especially where connections areprovided for each pixel in the array, are useful for providingelectrical communication between a touch screen sensor array and aseparate substrate that includes a control matrix for the array ofpixels.

FIG. 32 is a cross sectional view of another display assembly 3200,according to an illustrative embodiment of the invention. The displayassembly 3200 includes a modulator substrate 3202 and an aperture plate3204. The display assembly 3200 also includes a set of electricalinterconnects 3210 formed on the modulator substrate and a set ofelectrical interconnects 3212 formed on the aperture plate 3204. The twosubstrates make electrical connections between the interconnects 3210and 3212 by means of an anisotropic conductive adhesive 3214. Theaperture plate also includes a dielectric isolation layer 3211 and areflective aperture layer 3208. Not shown in this view are shutterassemblies formed on substrate 3202 or any aperture windows in thereflective layer 3208, since this cross sectional view is taken along aline closer to the periphery of the display, outside the array ofpixels.

The anisotropic conductive adhesive (i.e. “ACA”) 3214 is a polymeradhesive that is impregnated with a collection of solid conductingspheres 3216. When the two substrates 3202 and 3204 are compressedtogether the conducting spheres become trapped between the contact areasof interconnects 3210 and 3212. An electrical connection is therebyestablished between the interconnects 3210 and 3212 on opposingsubstrates. The electrical connection becomes locked in place after thepolymer matrix of the ACA is cured or polymerized. The conductingspheres 3216 can range in size from 5 to 50 microns in diameter. Thespheres 3216 can be made of a metal, such as nickel, or from alloys suchas Ni—Au, Ni—Ag, or Cr—Au. Nickel has sufficient hardness to maintainit's geometry under compressive loading. The conducting spheres can alsobe fabricated from dielectric materials, such as glass or polymer, whichare then coated with a conducting layer such as gold. In an alternateembodiment, metal spheres such as nickel can be coated with anothermetal with a higher conductivity and resistance to oxidation, such asgold, to reduce contact resistance.

When the conducting spheres are selected for uniform diameter, then thesqueezing of the contacts 3210 and 3212 to the conducting spheres 3216establishes a fixed spacing or gap between the contacts. The conductingspheres 3216 can therefore perform the same function as the spacers 2312or 2357.

The conducting spheres do not make a continuous contact along an axisparallel to the plane of the substrates. As a result, an electricalconnection is generally not established between neighboring pairs ofconductors 3210 or pairs of conductors 3212. Therefore a singleapplication of the ACA 3214 can be sufficient to make multipleindependent electrical connections between independent sets ofinterconnects 3210 or 3212. This particular interconnection medium isreferred to as an anisotropic conductive medium.

The ACA 3214 is generally applied by means of a tape or needle dispenseover multiple independent electrical contacts. It is particularly usefulfor making multiple connections along the periphery of a display. Forinstance it can connect together a series of row interconnects on onesubstrate to a parallel set of row interconnects on another substrate.This might be useful, perhaps, where the control matrix is built ontothe modulator substrate 3202 while driver chips are attached to aparallel set of interconnects on the aperture plate 3204. The functionsof the substrates 3202 and 3206 can also be reversed, so that themodulator substrate is on the bottom, closest to the backlight and thesubstrate on the other side of the ACA 3214 functions as a cover plate.

The electrically conductive spacers 3112 in display assembly 3100 arepreferably used in each and every pixel in the array, although they canbe applied on the periphery of the array. The electrically conductivespheres 3216, which are also spacers, in display assembly 3200 arepreferably applied along the periphery of the array of pixels. Thespacers 3020 of display assembly 3100, which perform as motion stops,are preferably used at every pixel in the array. The spacers 2312 or2357 of display apparatus 2300 and 2340, respectively, can be placedeither within or between each pixel of the array. Similarly the spacersdepicted in FIGS. 20 and 21 of U.S. patent application Ser. No.11/361,785, may also be included with each and every pixel in the array.

The association of a spacer with each and every pixel is not, however,necessary to maintain a desired gap between substrates. A spacer can beassociated, for instance, with each group of four pixels or with eachgroup of 16 pixels. In other embodiments the spacers might be restrictedto the periphery of the display. Alternatively, as depicted in displayassembly 3000, a display assembly can include multiple spacers perpixel.

The spacers 2357 in display apparatus 2340, for instance, areresponsible for maintaining an aperture to shutter spacing H6 that canbe as small as 1 micron. Denser spacer placing becomes particularlyuseful when display resolutions exceed 200 pixels per row or column, orwhen display diagonals exceed 2 inches. A dense array of spacers is alsouseful in display apparatus 2300, for example, for maintaining a uniformpressure on the lubricating fluid 2322, which might otherwise bedisrupted by local pressures, such as finger pressure, applied to thefront surface of the display.

The spacers described with respect to this invention can be usefullyapplied to maintain the gap between substrates in an electrowettingdisplay, such as display apparatus 2500 or 2600. Any display thatemploys MEMS-based light modulators, in fact, can benefit from spacersapplied within the interior of the array of light modulators. Examplesof alternate MEMS-based light modulators include digital mirror devices(DMDs), interference modulation displays (IMODs), and light tap displaysor frustrated internal reflection displays.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The forgoingembodiments are therefore to be considered in all respects illustrative,rather than limiting of the invention.

1. A display apparatus comprising: a first substrate; a secondsubstrate; an array of MEMS light modulators formed on one of the firstand second substrates; and a rigid spacer located within the interior ofthe array, integrally formed from or connected to the first substrate ata first end and connected to a second substrate at a second end.
 2. Thedisplay apparatus of claim 1, wherein the connection of the rigid spacerto one of the first and second substrates comprises a connection to astack of at least one thin film deposited on the substrate.
 3. Thedisplay apparatus of claim 2, wherein the thin film comprises one of areflective aperture layer, a light absorbing layer, and a color filter.4. The display apparatus of claim 1, wherein the rigid spacer is etchedfrom the first substrate.
 5. The display apparatus of claim 1, whereinthe rigid spacer is etched from a film deposited on the first substrate.6. The display apparatus of claim 1, wherein the first substrate andsecond substrates are substantially rigid.
 7. The display apparatus ofclaim 1, wherein one of the first substrate and the second substrate issubstantially rigid, and the other of the first substrate and the secondsubstrate is substantially flexible.
 8. The display apparatus of claim1, wherein the rigid spacer is formed from an insulative material. 9.The display apparatus of claim 1, wherein the first substrate includes afront surface of a display device.
 10. The display apparatus of claim 1,wherein the second substrate includes a front surface of a displaydevice.
 11. The display apparatus of claim 1, comprising a light guidedistinct from the first and second substrates.
 12. The display apparatusof claim 1, comprising a plurality of additional rigid spacers, whereinthe rigid spacer and the plurality of additional rigid spacers arepositioned within the array with a rigid spacer density less than orequal to one rigid spacer per four light modulators.
 13. The displayapparatus of claim 1, comprising a plurality of additional rigidspacers, wherein the MEMS light modulators correspond to respectivedisplay pixels, and wherein the each of the respective display pixelsincludes at least one rigid spacer.
 14. The display apparatus of claim1, wherein the rigid spacer forms an electrical connection between anelectrical component on the first substrate to an electrical componenton the second substrate.
 15. The display apparatus of claim 1, whereinthe rigid spacer is formed from a polymer.
 16. The display apparatus ofclaim 1, wherein the rigid spacer is formed from a metal.
 17. Thedisplay apparatus of claim 1, wherein the MEMS light modulators areconfigured to selectively obstruct the passage of light.
 18. The displayapparatus of claim 18, wherein the MEMS light modulators comprisesshutter-based light modulators.
 19. The display apparatus of claim 18,wherein the MEMS light modulators include electrowetting-based lightmodulators.
 20. The display apparatus of claim 1, wherein the MEMS lightmodulators selectively extract light from a light guide.
 21. The displayapparatus of claim 1, wherein the rigid spacer limits a range of motionof a component in one of the MEMS light modulators.
 22. The displayapparatus of claim 1, wherein the rigid spacer interfits with analigning element formed on the second substrate.
 23. The displayapparatus of claim 1, wherein at least one of the first substrate andthe second substrate are substantially transparent.
 24. The displayapparatus of claim 1, wherein the rigid spacer is between about 1 micronand about 10 microns tall.
 25. The display apparatus of claim 1, whereinthe rigid spacer makes contact to a heat-reflowable material.
 26. Adisplay apparatus comprising: a first substrate having a front-facingsurface and a rear-facing surface; a second substrate in front of thefront-facing surface of the first surface; a reflective aperture layerincluding a plurality of apertures disposed on the front-facing surfaceof the first substrate; and a plurality of MEMS light modulators formodulating light directed towards the plurality of apertures to form animage.
 27. The display apparatus of claim 26, wherein the firstsubstrate comprises a light guide.
 28. The display apparatus of claim26, wherein the plurality of MEMS light modulators compriseshutter-based light modulators.
 29. The display apparatus of claim 26,wherein the plurality of MEMS light modulators comprise electrowettinglight modulators.
 30. The display apparatus of claim 26, wherein theMEMS light modulators are disposed on the first substrate.
 31. Thedisplay apparatus of claim 26, wherein the first substrate istransparent.
 32. The display apparatus of claim 26, comprising a lightguide positioned behind the first substrate, wherein the reflectiveaperture layer reflects light not passing through the plurality ofapertures included in the reflective aperture layer back towards thelight guide.
 33. The display apparatus of claim 32, wherein the firstsubstrate is in intimate contact with the light guide.
 34. The displayapparatus of claim 26, wherein the reflective aperture layer is formedfrom one of a mirror, a dielectric mirror, and a metallic film.
 35. Thedisplay apparatus of claim 26, wherein the first substrate is positionedsuch that the reflective aperture layer is proximate the plurality oflight modulators.
 36. The display apparatus of claim 26, wherein thefirst substrate is positioned with respect to a light guide so as toform a gap between the first substrate and the light guide.
 37. Thedisplay apparatus of claim 36, comprising a fluid filling the gap. 38.The display apparatus of claim 37, wherein the fluid has a first indexof refraction and the light guide has a second index of refraction, andwherein the first index of refraction is less than that of the secondindex of refraction.
 39. The display apparatus of claim 26, wherein theplurality of MEMS light modulators are formed on the second substrate.40. The display apparatus of claim 39, wherein the plurality of MEMSlight modulators are formed on a rear-facing surface of the secondsubstrate.
 41. The display apparatus of claim 26, comprising a controlmatrix formed on the second substrate for controlling the plurality ofMEMS light modulators.
 42. The display apparatus of claim 26, whereinthe second substrate is transparent.
 43. The display apparatus of claim26, comprising mechanically interlocking features for maintaininglateral alignment between the first and second substrates.
 44. Thedisplay apparatus of claim 26, comprising an adhesive for maintaininglateral alignment between the first and second substrates to less than 5microns in any dimension.
 45. The display apparatus of claim 44, whereinthe adhesive comprises a heat-reflowable material
 46. The displayapparatus of claim 26, wherein the first substrate is positioned withrespect to the second substrate so as to form a gap between the firstsubstrate and the second substrate, the display apparatus comprisingspacers for maintaining the gap.
 47. The display apparatus of claim 46,comprising a fluid filling the gap.
 48. The display apparatus of claim46, comprising a liquid filling the gap.
 49. The display apparatus ofclaim 47, wherein the fluid is a lubricant.
 50. The display apparatus ofclaim 47, wherein the fluid has a first index of refraction and thesubstrate has a second index of refraction, and wherein the first indexof refraction is greater than or substantially equal to that of thesecond index of refraction.